I need to know if it is possible to register my game of Diablo II. When I click in the option, it sends me to a page that mention a 404 error, page not found. What can i do?
Where are you trying to register your game key? Where did the game key come from?
To claim a game key you normally go to Account Settings here on the website in the top right 
under your Battletag name. and where it says claim a code, you enter the code.
If you have an old 16 digit code from the original game CDs Blizzard converts that to a new longer key.
From there you can go to your games and scroll down to the Classic games to see that new code. Use that when installing Diablo 2 from the Blizzard Download page - also available in Account Settings.
If your key has been used or banned it won’t work.