I have just replied at other post with a probable cause of this. The presence funcionality requires UserEvents to be shown. That means you need the "user" permission on asterisk's manager.conf. But it seems that the latest trixbox has a patched asterisk that prevents the UserEvents from showing even if you have the proper permissions in manager.conf. In that case you have to add the "hud" permission to the manager user and the events will appear, and presence in FOP2 will work.
This only happens on this patched asterisk that comes with trixbox... and I think it is a bug, because FOP2 does not need any of the hud events that are being generated for working properly. The "hud" permission is also propietary of trixbox, it does not exists on asterisk.
Best regards,