HTC Touch Pro2 T-Mobile User Manual page 141

Note The information required to sign in will vary depending on the instant
messaging community you are accessing.
5. To log into a community, tap Sign In.
The instant messaging screen for the selected community will then be displayed,
and you can start chatting online.
For more information on using Instant Messaging, tap Options > Help and then
choose a help topic.
Exchanging Messages 11