On this forum page, click your name (upper right), Account Settings. You’ll have to redeem the keys for both Diablo 2 and the Lord of Destruction expansion and bind them to your Blizzard account.

If you purchased a box copy from a store or online store, it’s probably an old key. You’ll need to copy the 26 alpha-numeric code above “License Key” and paste it into the game client installer when prompted. “Diablo II Lord of Destruction” – “License Key” – “Download game client” “Diablo II” – “License Key” – “Download game client” Scroll down to “Classic Games”, and you should see this:

On this forum page, click your name (upper right), Account Settings, Games and Subscriptions. If you purchased your copy through the Blizzard store, it should be bound to your account.