Is Navy SEALs: Covert Operations ET free?

Yes. Navy SEALs: Covert Operations ET is released under Free fan mod (see the ModDB page for terms) and costs nothing. Neither it nor Wolfenstein: Enemy Territory has any paid content.

What do I need to run Navy SEALs: Covert Operations ET?

Wolfenstein: Enemy Territory, which is itself free. Navy SEALs: Covert Operations ET is an add-on and will not run standalone.

How do I install Navy SEALs: Covert Operations ET?

The PlayBound Launcher installs it in one click from Wolfenstein: Enemy Territory's mods list. You can also install it by hand — full steps are on this page.