Is Hell in the Pacific Total Conversion free?

Yes. Hell in the Pacific Total Conversion is released under Free fan mod (see the ModDB page for terms) and costs nothing. Neither it nor Medal of Honor: Allied Assault War Chest has any paid content.

What do I need to run Hell in the Pacific Total Conversion?

Medal of Honor: Allied Assault War Chest, which is itself free. Hell in the Pacific Total Conversion is an add-on and will not run standalone.

How do I install Hell in the Pacific Total Conversion?

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.