Redeem a TESO key is really simple, just follow those steps:
1/ Login into your The Elder Scrolls Account and go here
2/ Select "REDEEM CODE" on the right side of the page
3/ Write your code and click "REDEEM CODE"
4/ Pick your membership plan and click "Continue"
5/ Your code should be activated on your account
The key is not working
If you have activated the key according to the guide but the code appears not to work, please check what type of error message you receive and depending on it follow the appropriate instructions: