Is HaZardModding Coop free?

Yes. HaZardModding Coop is released under Non-commercial fan project (see hzm_legal.txt in the download) and costs nothing. Neither it nor Medal of Honor: Allied Assault War Chest has any paid content.

What do I need to run HaZardModding Coop?

Medal of Honor: Allied Assault War Chest, which is itself free. HaZardModding Coop is an add-on and will not run standalone.

How do I install HaZardModding Coop?

The PlayBound Launcher installs it in one click from Medal of Honor: Allied Assault War Chest's mods list. You can also install it by hand — full steps are on this page.