I was going to ask what browser you are using. I've heard of these issues with Safari and Chrome. From the help center:
Macintosh/Safari users, please read this!
If you are using a Macintosh running Safari, you may have some difficulties with several features of the PBS site:
Some Safari users report odd display issues
Most Safari users are not able to apply changes that require text entry
such as forum posts, profile changes and Requestor Conditions
Some versions of Safari have incompatibilities with the PBS site. These are due to changes in the Safari browser code, and are out of PBS' control. The latest Safari updates (for Leopard OS 10.5) are working well; earlier Safari versions (for Tiger, OS 10.4) those Mac users who do not have Leopard or Tiger will need to choose another browser in order to have full use of the PBS site. No matter what version of Mac OS you have, you should download Apple's free software update (click Software Update under the blue apple on the top left of your screen).
To enter text into your public profile, forum sig, Requestor Conditions, or to Post in the forums if you have a Mac running an earlier version of Safari, you will need to use another browser. We recommend downloading Firefox, which is a FREE browser available here. Firefox is the optimal browser for viewing and navigating the PBS site.