If you have Mountain Lion you don't (you have Messages)
iChat comes with the OS in any version back to iChat 3 (earlier than that it was a Download.)
Version prior to iChat 6 in Lion can be "reinstalled" using the Install DVDs
Lion needs to be Reinstalled to get iChat 6 "back".
Other than that I am going to be guessing what OS you have.
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