Note: to use talk.google.com, you need a gmail account. I THINK you can get one without an invite now; try hitting up https://www.google.com/accounts/NewAccount and see if you can create a Gmail account from there, if you need to.
Here's this dude's instructions on how to set up google talk on Gaim (or, from what I understand, Adium), since the instructions for doing so AT talk.google.com are b0rk3n:
1. Add an account, select "Jabber" as the protocol.
2. Your screen name is everything before the '@gmail.com'.
3. Server is 'gmail.com'.
4. Click "show more options" and make sure "use TLS if available" is checked. Leave "Force old SSL" and "allow plaintext..." unchecked for now. Connection port should be 5222, connection server should be 'talk.google.com' without the quotes.
That's it - you're good to go, log in. Don't forget the "@gmail.com" when you add people - even though it's ALWAYS "firstname.lastname@example.org", it won't work without you specifically typing the @gmail.com when you add them. I'm on there as jimbosworldorg - again, don't forget the @gmail.com on the end.