Hi,
If you are starting out afresh then the account can be set up in Messages (and it will get added to Internet Accounts in System Preferences)
Then it changes to allow you to select AIM or Jabber
Select Jabber.
Enter the Jabber ID (You have to have one first) and the Password.
If in System Preferences > Internet Accounts you have a Jabber ID already set up it may need activating for Messages.
This is a view of my Google account in Internet Accounts. It is active for Mail and Messages
As you can see from my first picture accounts show in the list in Messages (you cannot delete the iMessages and Bonjour ones).
If in the General Section of the Preferences you Untick the item that gather all the accounts into one list (That is everything but iMessages) they will display separately with individual Buddy lists.
They then also show in the Window Menu with their own Keystrokes (if you don't have more then nine)
They also show bottom left of the main chat window.
9:12 p.m. Monday; September 7, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
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