Google Talk Password in iMessage
I, too, had this same problem until I did the following:
1. Sign in to your Google account
2. Go to "Authorized Access to your Google Account" page. Lower down on that same page you will find "Authorized Access to your Google Account". There is a tutorial there to watch if needed.
3. In the box provided type a description of the application, e.g., "Google Talk on my iMac" and then click on "Generate Password"
4. It will then generate for you a password that you copy and paste into iMessage. It is a one-time password that you don't have to remember and can be revoked if you want to set things up differently in the future!
This solved the problem for me. Hope this helps!