Originally Posted by purplecatty

Hey! It's not in BB's CD that came with BB. You'll have to download a separate software that is for connecting BB to PC for internet purpose. For example: My BB Curve 8330 connect to laptop using SprintSpeed software that I had to manually download from Sprint. It was not included in BB Curve CDrom that came with my BB..

Tmobile may have it's own Phone As Modem software. I did this by google searching. Here it is, scroll it halfway down and you should see "Tmobile's Phone as Modem plan" and software is called "ActiveSync"

Here it is ..
Use Your Cell Phone as a Modem for Your PC

There's another one, you might be interested more updated version:

How To: Tether Your T-Mobile Smartphone as a Modem (2008 Edition)

Good luck
Catty