Q: facetime wont let me sign in

For one thing, you must be on a WiFi network to use FaceTime from your iPad.

Check your FaceTime settings.  FaceTime should be on, your Apple ID should show there correctly, and "You can be reached for video calls at" should be set to your Apple ID.  Your Caller ID should also be your Apple ID.

Has FT worked before?

Using FaceTime http://support.apple.com/kb/ht4319

Troubleshooting FaceTime http://support.apple.com/kb/TS3367

The Complete Guide to FaceTime: Set-up, Use, and Troubleshooting Problems

http://tinyurl.com/32drz3d

 Cheers, Tom

I'm surprised you don't see a FaceTime on/off (and a "you can be reached ...") option.  That's the option at the top of my iPad's settings (iOS 5.1.1).

Are you in the U.S.?

As this article says: http://support.apple.com/kb/HT4319

"FaceTime may not be available in all countries or regions."

Might that be your issue?  It sounds like it.

Try going through the Troubleshooting FaceTime steps in http://support.apple.com/kb/TS3367