Firstly, you must have your m3u file downloaded
Now, on MrMC go to:
Settings > TV > General > Enabled.
It will tell you there is no PVR add-on enabled, click OK.
It will now present you with a list of PVR clients. Scroll down to "PVR IPTV Simple Client", click it.
On the General tab, change the location to either local path(if the m3u file is located on your FireTV/FireStick/Shield) or remote path(if its hosted on the web), and then locate your m3u file
Now click OK, then Enable. Then reboot your box.
After restarting MrMC, You will now have a TV section on Home Screen
Go inside that and all your channels should be loaded.