hi Kami, I'm sorry but I have to tell you that I don't really know how to make the fullscreen button work... I started a new cms project for my company two years ago, so I had to drop WB.

Anyway, if I'm not wrong, there were some issues with fullscreen using the older versions of jwplayer, maybe upgrading to a newer one should do the work... (you just need to replace ufo.js and flvplayer.swf)
tell me if you can make it work this way, so I'll update my original post
Emanuele