I am only familiar with the Raspberri Pi interface, but one thing that comes to mind .... when you say you installed the MythTv PVR, was it a client you installed? (was it under a client menu?) Then I would say you have the client, but not the backend server, which the client needs. The backend can be on the same machine, if it can be installed on that machine, and the machine is powerful enough to support both front end and back end at the same time.
I use the tvheadend PVR.
I have installed and run mythtv before, but never tried to get it to recognize a capture card. Mythtv uses mysql, which sometimes does not gracefully start at first depending on machine and installation.