jpl,
The modem driver should be automatically installed already with your 7100t. If not, use the BlackBerry desktop that came with your T-Mobile 7100t (since not all versions of Desktop 4.0 has the BlackBerry modem driver). If you are already using the T-Mobile BlackBerry Desktop, try upgrading to 4.0. Also, T-Mobile port blocking may make this issue somewhat problematic.