Do everything what Spencer recommends with the Ophidian service book.

If you're having the same problem as me once you have your Y! messenger running and having requested your Service Book, your custom Email account Service Books are missing. The way to get it is through the Email Setup (and not through the bis website).

I also just talked to the 'Data Service Customer help' and here's what they gave as an alternative to access Yahoo, AOL, Windows Live:

1. Go to Media Net
2. Navigate to Mail and Messaging > Instant Messaging
3. Download, and then Run the application.
4. Should have an icon now with '2 people' - blue and gray
5. Clicking the icon will allow you to connect to either of the 3 messengers.

Hope this helps. It worked for me.