Just go to the Hauppage support web site and download version 4.6 of the installation CD for WinTV and the drivers for the 1250. This CD contains a folder called "MCE_soft_encoder," with the file SoftMCE_Setup.exe. Run that setup file, and you should have no problems using MediaPortal with your 1250 card.
This is the version of the CD I have and the file I try to run. I think my PVR150 is confusing the software, and I don't care enough to take out my card so as to install the encoder.