I have the exact same issues as you describe. The RIM Virtual Serial Port v2 was not automatically installed when I installed Desktop Manager 188.8.131.52. When I tried to manually install the virtual serial driver (Add hardware...) it appears to add the port but then display the error.."the system cannot find the file specified".
I have gone through the uninstall, reboot, install cycle and also tried the repair option...no luck. I have tried v4.1 as well...but again no luck.
Getting my BB to work as a modem would be a huge benefit for me, so any assistance would be much appreciated..