I've had a 5-button mouse for about a decade and use this extensively for games on both Mac and PC (bootcamp).
However i can't seem to allocate any functions to button 4 or 5 - although the game use to recognise these as different from MB3, they would all perform the same action as MB3. Now i can't even get the game to recognise these as separate buttons.
I run ESO on bootcamp as well and have no problems in that respect, all 5 buttons perform distinct actions.
Is this a known issue? Or is there something i can do to fix?