Well, like I said I'm fairly new around here too, plus I have cingular (att), but I'll try to help as much as I can.
I first logged into my BIS account via
www.cingular.com/blackberry (I'm not sure what tmobiles web site is). I then removed my current email account. Then I told it I wanted to setup a new account on the web site, I then deliberately typed in the wrong password on the first screen and hit next. It then gave me the option to provide the settings to add this email account. So I selected that and went to the next page. On that screen I selected work account and on the next screen it allowed me to choose whatever protocol worked best for me.
I'm sorry this may sound confusing but it's actually really easy once you login into the BIS web site. There also may be an easier way to do this, but that's the only way I know right now.