After you have launched Messages it will insist on setting up an iMessages account (this is the iOS service that has been shoe-horned in to iChat)
After that has been done the Accounts List In Messages Menu > Preferences > Accounts should also show yur old "iChat Accounts" as well if you Upgraded Lion to Mountain Lion rather than A fresh Install.
These will present as a Combined Buddy list (See Window Menu > Buddies) but can be made to appear as separate Buddy lists by changing the setting in Messages > Preferences > General Section. (Untick the line about gathering all Accounts in to one list)
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
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