Follow the instructions below:
1) Go to www.battle.net
2) Click on "My Account" on the top right of the page
3) Create a new account or login
4) Once logged in, go back to www.battle.net and click on the top right of the page where is should state your name or username then click on "Account Settings"
5) On the left of your account page, click on "Download Games"
6) Scroll down and find the games then click on download.
7) During installation, it will ask to enter the product key.