Hey Beeker,
Are you trying to get EPG from over the air? If you go into Mythtv setup, under the video sources there is a field called grabber (or similar) that needs to be set to EIT.
Once that is saved the system in theory should populate guide data for you.
I'm not in front of my system so I cannot check the field names etc but can check tonight if you cannot find it.
Cheers,
Kezza