Zen is a branch of buddhism. Buddhism is based around the Buddha's "Four Noble Truths". The first truth is that life is suffering, the second truth is that the reason for suffering is attachment. The third truth is that suffering can be overcome by eliminating attachments, and the fourth truth... um, I forgot that one. It was probably not that important and a bit silly.
Anyways, in addition to the four noble truths, zen emphasizes the direct and authentic experience of enlightenment. Non-dogmatism seems to be most central to zen. Quoting an authority (the buddha himself):
"Do not believe in something, just because people think highly of it, or because it has come from tradition, or because it is found in scriptures. Consider whether it goes against your judgment, whether it could cause harm, whether it is condemned by wise people, and, above all, whether put into practice it will bring about destruction and pain. Anything that you judge to be beautiful, accords with your judgment, is appreciated by wise people, and, once put into practice, will bring about joy and happiness, can be accepted and put into practice."
Thus, by the buddha's own words, the consumption of psilocybin mushrooms is definitely allowed for a practitioner of zen. In fact, I would argue that every method which can give you zen enlightenment is a valid practice of zen. In an age of technology, this could mean (to give a somewhat whimisical example) uploading your mind into a computer, manipulating it with "Microsoft Mind-Editor" into a state of enlightenment, and then downloading it into your brain again. That, I think, would be in the spirit of ancient zen masters, who were always trying to discover a new and better method for achieving enlightenment. They even came up with a meditation practice that is unique to zen, namely the application of koans. A koan is a question that cannot be solved by the rational mind, something like "What was your face before birth?". You meditate on a koan during your whole waking time, no matter if you are sitting or walking, not thinking about anything else but the koan, attacking it with all your mental powers, until, one day, your mind breaks and you become enlightened.
What, then, is zen enlightenment? To answer that question, one has to look at what enlightened people do. Here's an example.
In the 18th century, there was a zen master named Hakuin. Hakuin led an ascetic life in a small village, and he was highly respected by the village people. Everyone considered him a holy man. Then, one day, a girl living next to Hakuin got pregnant, and, when asked, she said that Hakuin was the father. The village people became very angry, went to Hakuin, gave him the baby, and said that, since he was the father, he would have to care for the child. All that Hakuin answered was: "Is that so?". Hakuin took the child and cared for it like a father does. He begged to obtain milk and food, and, amidst of all the people who were insulting him, spitting at him, and physically attacking him, he did the best he could do for the child. Meanwhile, the girl could not stand what was done to Hakuin, so, two years later, she told the truth. The real father was a guy from the fish market. Again, the village people went to Hakuin and told him about the mistake. All that Hakuin answered was "Is that so?", handing back the child.
That's zen. A zen master is a very cool thing to be. And the best thing is that you can be a zen master, too. To become a zen master, you have to achieve zen enlightenment. What, then, is zen enlightenment? First of all, it is an experience. How would one describe an experience? How would you describe the experience of love? I think that, for the purpose of communication, one could give a definition of love, but even when you fully grasp that definition, you would have no idea as to what love really is. In the same way, it is probably impossible to understand enlightenment for someone who has not experienced it (and I haven't), but, nevertheless, one could give some characteristics of enlightenment, which seem to be common.
1. Zen enlightenment takes but an instant. It is a sudden experience, not a gradual one. That seems to be a major difference between zen and, for example, yoga.
2. After achieving zen enlightenment, one is able to live completely in the present moment.
3. Someone who has achieved zen enlightenment has absolutely no fear of death whatsoever. He enjoys life, but he accepts death.
4. Upon achieving enlightenment, you are able to act freely, not hindered by any definition of yourself. You have lost your ego.
5. When you achieve zen enlightenment, you do not spend your life on a mountaintop or surround yourself with flowers. You go back into the dirt and mud of real life, and actually achieve things (which means helping people). The ideal of zen is the Bodhisattva, he who is able to enter Nirvana, but does not. The Bodhisattva reincarnates and reincarnates until every sentient being in the world has become a buddha. That's a major difference, for example, between zen and most of the new age crap.
So, how does one find enlightenment? The main point seems to be to keep searching. Although meditation is probably a quite reliable method for becoming a buddha, sitting down each day for one or two hours of peaceful meditation is definitely not enough. To quote Hakuin out of my head (I can look up the exact quote, if someone is interested): The man who looks for his real nature is like someone who has a valuable gold coin in his pocket. When walking in the rain through a marketplace full of people, he suddenly discovers that he has lost the coin. What would he do? He would not peacefully sit down and sift through the sand, looking for his gold. Nope, he would scream and rage and push people aside, and, with tears flowing down from his eyes, dig in the mud and dirt until he has found again what belongs to him.
Want to be a zen master? Search. Keep moving. Because, once you settle down, once you stop moving for even just an instant, you won't know if you will ever be able to move again.
That was lengthy. Hope it helps.
DISCLAIMER. 99% of zen practitioners will disagree with my definition of zen. But zen is just a word, so in order to avoid pointless debate about definitions, I would like to admit that, above, I did not really give the definition of zen, but the definition of nez (thanks, Mr_Mushrooms). But becoming a nez master is a cool thing, too, so go for it!