mp_category pages not real category / archive template?

Having some major issues with my mp_category page. What exactly does marketpress do to indicate to wordpress that the mp_category template should be used for those custom taxonomy pages?

Trying to narrow down why none of my category queries work properly and as far as I can see, wordpress doesn't think that it's actually a 'real' taxonomy page, so a bunch of standard functions / variables aren't present and/or don't work as intended.

How does marketpress tell wordpress to use this template?