The displacement map generator creates a map based on the differences between the current subdivision level and the highest level. You’re probably still on the highest level, which means that there are no differences.
I love the new website, I’ve only been on it five minutes and already I have found the answer to two of my most pressing Zbrush questions, one being how to use AUV Tiles correctly with an imported mesh and why nothing happens when I press the displacement map button. For the longest time I thought it was a lack of ram so I upgraded from 512mb to 1gig and still nothing happened when i clicked (which made me really depressed) so thanks.
Found the solution already.
Thanks!
cant start a thread to post a new question so i ask it here 
anyone knows how to reaply a displaccement map back in zbrush?i know it has something to do with re-importing it as an alpha but cant find how to apply it to the model…any help would be welcome
tx