Log in (or sign up) with Battle.net.

Activate the game on your Battle.net account by clicking the "Activate with Battle.net" button.

The game will be added to your Battle.net account and becomes available through the Battle.net desktop client.

As for the installation please check here: https://us.gamesplanet.com/community/call-of-duty-modern-warfare-standard-edition-download--4252-1/discussions/180893-important-how-to-install-cod-modern-warfare-on-your