Then PLEASE use a supported capture card.
In your case, since you insist on using USB, you have two options:
* Hauppauge HD-PVR
* Hauppauge PVR-USB2
These devices have Device Templates made for LinuxMCE which configure them for use with MythTV.
Since you're not reading instructions, I do have to ask, have you built your first LinuxMCE machine with two NIC? or used NVIDIA for your GPU?
In any case, Look in the wiki under supported tuner cards.