Hi,
In the Address Book, or Contacts app if you are using Mountain Lion, you can add AIM, Jabber or Yahoo Screen Names and IDs to the Instant Messages section.
In iChat or Messages (Mountain Lion Again) you can add accounts by using your own AIM Screen Name or a Jabber ID or a Yahoo ID.
You can do it the other way round.
Start up iChat or Messages, add an account, use the Buddy list to add a Buddy then link that to an existing Address card.
iChat can send SMS.
However there are Limits.
The computer must be set to be in the United States (System Preferences > Text and Language)
The receiving phone must be in the United States - on a carrier that supports the AIM SMS forwarding.
List
You then add the phone Number like it was the Buddy's Screen Name to the AIM Buddy list.
It must have the +1 United Stated Country code added to it as in +1234567890
The app will sort out the formatting of the number.
9:47 PM Tuesday; June 25, 2013
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously