I did the following and it works great!
Google "google talk blackberry" and the first one that should come up is from blackberry.com site slash googletalk. I'm not allowed to post yet, so just find the link via google.
When the email arrives on your bb, then dl it from the link on the site and it will automatically load onto the device.
You don't even need it connected via USB this way. After that, an icon will automatically appear.
Is it possible that the icon is hidden? You can go to ORGANIZE APPLICATIONS and check to see if it's hidden. Just a possibility *shrugs*.