It's an integrated application. If you go into the Control Panel and choose "Accounts" you can set up the information for your GTalk account. Then it will connect and you'll see the colored status circle in the dock at the top of the screen.

Once you have the account setup/connected, you can make calls by just choosing "New Internet Call" from the main application menus or by going into your contact list and choosing one.

This goes for any of the build in protocols in OS2008 (GTalk, Jabber, SIP). The beta version of the communications suite adds a bunch more (ICQ, MSN, AOL, etc) but they are all text chat only.