I don't personally have this card, but I did do some research a while ago when I was thinking about buying that particular card and I remember reading somewhere that someone has worked out how to get the analog AV input to work in mediaportal.
I have the following links bookmarked so there may be some details on how to do this in one of these:
Media Portal and Hybrid Tuners Help How to get analog TV working with DNTV Live Pro and MediaPortal - DVB Owners Discussion Forum - dvbowners.com
Hope that helps.