Disregard the first post I entered here. I think I figured it out. In the Date & Time window, on the Date & Time tab, click on the Open International button.
Or just go to your internation preferences. Under the Formats tab, click on the customize button for Times. Here, chose the 12 hour clock, You should see the AM and PM for the before Noon and after Noon settings. That should make the Show AM/PM box selectable in the Date & Time preferences again.
Wierd, if you have 24 hour clock set under international times preference, you can still toggle the 24 hour clock in Date & Time, but can not toggle the Show AM/PM.