Battle.net Activation Guide
How to redeem a battle.net code.
Online (recommended)
Step 1: Log into your Account Overview page
Step 2: Enter the code in the Redeem a Code box and click Redeem Code
Via the app
Step 1: From the Games tab, click the Redeem a Code button found under the logo of any game
Step 2: From the Shop tab, click Services and then Redeem a Code