Hi,
System Preferences > Mail, Contacts and Calendars
This Preferences Pane may well list your "iChat Accounts" and have them Enabled for Messages.
Of they are they should appear in Messages menu > Preferences > Accounts.
They may need to be Enabled here.
IF you have several AIM or Jabber Names and IDs or you have the Bonjour Account Enabled then the defaults is for a Combined List (See Window Menu > Buddies (CMD + 1) )
You can unlink the Buddy lists by going to Messages > Preferences > General Section and unticking the line about gathering all the accounts in to one list.
This will result in the Window Menu list all the Non iMessage Accounts with their own CMD + #
As you have probably worked out the "Messages Window' is both Chat window and interface.
It has a New Message icon (Square box with Pencil) top left
After you use this a Blue + iCon appears top right
Using this gives you access to Contacts (Formerly Address Book) directly, Groups in the Contacts List and Buddies
This Buddies drop down is all your Buddies from all your Accounts that are On Line.

8:47 PM Wednesday; September 26, 2012
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 and iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously