The Mac Messages beta application uses e-mail addresses (since most Mac's do not have an associated phone number).
Using my iPhone Messages app; How do I enter an e-mail address to address a new message to aMac Messages beta recipient?. The iPhone Messages app keyboard only displays alphabetic characters and numbers, no symbols, like the 'at' sign ( @ ) and the period ( . ) that are required in most e-mail addresses.
Also, the iPhone Messages app To:
field Plus sign ( + ) allows one to select a recipient from the Contacts app, but it only shows phone numbers, no e-mail addresses.
I have used the Siri voice recognition to spell out the special characters to compete the e-mail address and I suspect copy / paste would also work but there should be a way to address a new message to an e-mail address using the Message app keyboard.
Here is a link to images of the iPhone Messages app To: field keyboard to show that it is configured to not include special characters.http://gallery.me.com/vcina#100895
Of course, I could be missing some simple way to enter the needed e-mail special characters using the keyboard on the iPhone Messages app keyboard.