If you have an external numeric keypad, and it's detected in OS X but doesn't work correctly -- beep sound when used with iCal; closes the tag window in iTunes instead of typing year or track number; etc. -- here's a potential solution.
Press and hold Num Lock key on your numeric keypad first, then connect it. You can then release the key. Press it again if the Num Lock status LED on your numeric keypad isn't on. It should work perfectly at this point.
If this doesn't work, another tip I read on some forum somewhere is to try to press the Num Lock key on your already-connected numeric keypad for more than four seconds.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080310030635237