Verify that you added the correct site and theme in the Site Setup section under Settings. If you are trying to use PokerStars, the problem is most likely that you have modified the graphics.
Make sure they are selected in the main window. Some antivirus programs can prevent TT from working properly without adding an exclusion.
Follow this tutorial to verify that the site you play on is setup correctly. Then, contact support. If there were any problems with the setup, let support know.
You can contact support on the 2+2 forums.