How to initiate a conversation on google talk using the messages app? (I have already added my account and enabled it in the preferences menu of the messages app)

After upgrading to Mountain Lion recently, iMessage (through the messages app) works even better. But because the messages app has now completely replaced the former iChat app, it also has the ability to add google (google talk), yahoo and AIM accounts for instant messaging. I have added my gmail account through the prefernces menu in the messages app and have also enabled it. However I'm unable to start a chat with any of my contacts on gtalk. Once I start a new message and type in the concerned person's google email id (which is on my friends list, added through google talk earlier), I get a message saying the person isn't available on iMessage. I don't see any other option to open a new chat window for my google talk contact who are not on iMessage. Is there anything else that can be done or is it only possible to receive messages on google talk via the messages app on Mountain Lion?