How to send and receive iMessages from my phone number on my MacBook Air with Mountain Lion
This is simple to accomplish if you have an iPhone to associate your phone number with your Apple ID account. I'm not aware of any way to hack, trick or cajole Apple to add a cell phone number to an Apple ID other than it going through the normal activation process on iPhone only.


Activate your Apple ID - http://support.apple.com/kb/TS4268
Open iMessage on the Mac and in preferences, set the caller ID to initiate messages from your phone number instead of your AppleID email.




As you can see, it is allowed to add more emails to an Apple ID, but the adding of phone numbers happens when you activate things from a live phone account and SMS messaging verifies that the phone number matches the iPhone.

Once that's done, Apple is happy to let your Mac or iPad or phone "iMessage" from that telephone number.