I need some help with 'Registering' my mac...

Ok, I go to System Preferences > MobileMe, I can log in, but If I go to Sync, the box is unticked, and when I tick it it just goes to the Registering page and says "Looking for registered computers" and it finds nothing. So I click "Register This Computer" to register my iMac and it does the spin wheel for a few minutes, then just doesnt show my computer on the list, and when I tick the box on the Sync page the option of manual, automatic and etc is greyed out and I can't change it, and at the bottom it says 'Status: Registering Computer' but It won't register as I descriped...

Can anyone help me out with this or please tell me why It won't register my computer...