In case you are using Trillian 3 Pro and want to chat with people using GoogleTalk you can configure this in Trillian via the Jabber plugin. First you have to activate the Jabber plugin (in case it is deactivated) and then go to "Connections->Manage My Connections->Add New Connection".
Choose Jabber and enter your Gmail account details: This is your complete email address, e.g. firstname.lastname@example.org and your password.
After this click on Change ... and fill in the values shown in the following screenshot: