What version of honeycomb are you running on your tab? I don't actually have a 7.7, so I'm not sure what specific version it comes with. From memory, game controller support wasn't added to honeycomb until 3.2.1 or something like that. It didn't actually come with the initial version of 3.2, which is what I think the tab 7.7 runs when stock. I may be wrong about this, might be worth googling to check.
there are probably apps out there that can load xbox controller drivers for you though. Check the marketplace.