I was seeing this in the mythtv-setup log when trying to get it to detect tv_grab_nz.py:

2013-04-27 16:05:07.772632 I XMLTVConfig::Load: Running 'tv_find_grabbers baseline'.
2013-04-27 16:05:08.068895 E XMLTVConfig::Load: Failed to run tv_find_grabbers

So turns out you need the xmltv package to get the 7 day epg working using tv_grab_nz-py from here
Luckly, someone has already created a QPKG for xmltv on the QNAP here