Looking at the some web searches on that boss it looks like there may be problems getting the right bar to appear.
2 Sep 2012 -
http://us.battle.net/wow/en/forum/topic/6490009164 - When Tharon'ja transforms the group to undead, the special action bar does not appear.
14 Feb 2013 -
http://us.battle.net/wow/en/forum/topic/7922982721 - Tharonja removed all my abilities
30 Dec 2012 -
http://www.curse.com/forums/games/wo...itching-during - On the last boss of Drak'Tharon Keep, where you get transformed into a skeleton, your first action bar should switch to a new set of abilities. Mine doesn't.
Reply on the last one .. 'With both BT4 and Dominos, there is an option per action bar to switch with vehicle, but I believe it is off by default. In any case, I'm fairly certain that's what you want. '
So sounds like there is another macro action bar setting that needs to be set for this type of thing that might be hidden somewhere in Bartender and Dominos.