Open up Pidgin and choose Accounts –> Manage Accounts.
Then click the Add button.
Then you’ll want to enter in your username as the beginning of your email address, and the Domain as the part after the @ symbol. For instance, mine is geek@howtogeek.com, so I’m using geek as the username and howtogeek.com as the Domain.
Then flip over to the Advanced tab, and enter talk.google.com as the Connect server. You can pretty much leave the other settings alone if you want.
At this point you can click the Save button, and then you should be able to enable the account via the Accounts menu.