In System Preferences, go to Keyboard & Mouse > Keyboard Shortcuts. At the very bottom, there's an option for "Full keyboard access". Set that to "all controls" and it should work just fine.
This is one thing that annoyed me for a while after reinstalling my OS. I had somehow managed to apparently hit the shortcut (ctrl+F7) and had it working originally, but after the reinstall couldn't figure it out. I asked here and someone helped me figure that out. I can't live without it.
