(Applies to: all versions of FusionBB)
If you've just upgraded to a newer version of FusionBB, then find that you aren't able to turn on various forum permissions (for instance, allowing HTML posting), make sure that you resync the permissions for your site.
Permission boxes not being checked is a primary clue that they haven't been resynced (created in the database yet).
(This is a commonly overlooked step! ;) )
If you have any further questions, please post them at the main FusionBB site.
(Thanks to FusionBB member PatrickD and FusionBB Developer Josh Pettit for this information!)