It looks like FengGUI is dead or dying, the last update/post/news is from January 2009....3 years ago!!!
May, not January.
FengGUI was still being developed past that - I was submitting patches until about Jan 2010, when I left the company where I was using it, and there were a handful of other active forum users. But it did suffer from a main developer (possibly the previous one - my memory's hazy on the details) who didn't know how to use SVN branches, so decided to start a massive refactor in the trunk and then left it half-finished.
There should be a few examples which demonstrate how to use it with LWJGL, but if there's another library which meets your requirements I would try that first.