Mocha HitFilm is included with HitFilm Pro and is available as an add-on component for HitFilm Express.
When you install HitFilm Pro, Mocha is automatically installed, but will not be visible in the effects panel until the software is activated. If you are using HitFilm Express, you will need to purchase the add-on linked above, then restart HitFilm Express so your license can be updated from your account and the new effect will appear.
Mocha HitFilm can only be launched from inside HitFilm itself. You can find it in the Effects panel, in the Boris FX Mocha folder. Open that folder and drag Mocha HitFilm onto any video clip on your timeline. You can then use the controls for the Mocha effect to open the Mocha interface.