I have this very frustrating issue with iOS dev center. Every time I want to add a new iOS-device by its UDID the site gets stuck in the loading screen. This same problem occurs when I want to change an existing distribution profile. The site just goes "Loading" forever.
Considering that this occurs on all my devices (computers, mobilephones etc.) I would be very grateful if someone had any idea how to deal with this problem..
Are you using Safari? For some reason the dev site doesn't always play nice with other browsers like Chrome and Firefox