I've experienced the "greyed out" issue in Snow Leopard. I usually find the problem to be when I've selected a folder that doesn't exist in the previous backups. And, for the same reason, if your backups are from a different machine that also can be the problem. In either case you should be able to find your way around it, somehow. I may be wrong though.
If you want to restore to an hour ago you can do that, but you've got boot into the Lion installer and do it from there. I'd recommend backing up all your important data first though. Restart and hold "Option" when you see the white screen until you see a boot menu and select the drive that reads something along the lines of: "Recovery Drive".
If you're using Snow Leopard, or Leopard, rather than Lion you can insert the DVD that came with your Mac and restart and hold "Option" when you see the white screen until you see a boot menu and select the DVD.
In either scenario, if you see the Apple logo you've missed it and you'll need to restart and hold "Option" again and hope it works. Use the "Option" key on the left side of the keyboard.
Hope it works out for you.