how do i register a mobile number in iMessage

Howdy anthbur,

It sounds like you are wanting to link your phone number to your iMessage account. The following article will help you get that setup:

iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage - Apple Support

Thank you for using Apple Support Communities.

Hi,

Did you mean link as Sterling suggests or do you mean you want to contact or connect to an Mobile Number ?

If it connecting to someone who is not in your Address Book/Contacts app you can type directly into the "To" Spot.

You will need to add the "+ country code" ID numbers to the front of the number - as in +1 for the United States.

If you mean you want to connect to a phone number using an AIM account that can only be done to Mobile phones that are in the US.

If you have an AIM account they can be added as a Buddy but using the Same +1(number) as a Buddy instead of using a Screen Name. They do have to be on the "right" carrier. (AIM have a list somewhere)

The service delivers it as an SMS message.

10:05 p.m. Monday; October 12, 2015