I am still waiting for iOS 7 to finish downloading.
You have to have Mountain Lion as well.
The Messages app needs to have the same Apple ID the iPhone is using enabled.
On the iPhone this used to be Settings > Messages which has to be enabled first then have the Apple ID added.
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