The Computer needs to be set up as if it is in the Untied states from the various SMS items is several of the iChat Menus to appear.
You have to have an AIM account (Certain Apple IDs are also Valid AIM Screen Names).
You needs to add the phone your are calling as a "Buddy".
However this only works to Mobile or Cell phones that ar in the United States using Carriers that support the AIM SMS Forwarding.
The option in the File Menu does allow you to SMS to a Phone Number not in your Buddy List
It should be ALT + CMD + N to activate the Pop up.
This includes a Drop down for which AIM account you are using and a Space for the telephone number.
The Leading 1 (for the United States) is already shown along with a reminder in grey text that is is US Numbers only.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
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