Tvheadend

Hi guys, I'm sorry for asking what might sound like a stupid question, but I'm new to the whole concept of IPTV and would like to learn how to properly integrate it within tvheadend.
My setup is as follows, tvheadend running as a backend server on a raspberry pi, linking to a front end on another pi.

I've already setup DVB-T channels and completely happy with that but I can't seem to make sense of adding IPTV streams.
I've added a new network, and I've then attempted to add an IPTV stream to the MUX. But it never seems to pick anything up.
What exactly am I doing wrong?
Thanks for any help in advance.
Adam