Once you have configured Google Talk as the “System” of your agent in the “Agents” tab of your configurator and clicked the “Save” button, the “Visitor” contacts are automatically sent through to your Google Talk account.
To use Google Talk on your computer you can use the native Gtalk client that Google offers, or one of the other chat clients listed here:
You will receive four contact invitations from email@example.com, firstname.lastname@example.org, email@example.com, and firstname.lastname@example.org
If your contacts are appearing as offline you should hover over the contact or click the menu on the side. Here you should see a way to unblock or unhide (Auto should be selected):