First, you'll want to open up Adium.
Go to File > Add Account > Jabber
Then in the Basic tab, fill out the following:
Jabber ID: <APU Email address>
Password: Your current APU password
Connect Server: talk.google.com
The final two options it is recommended to leave with the default settings. By default Adium is set to Connect when Adium opens to APU Gtalk.