It doesn't work for me either.
I think you have to enter your DII account name instead of your Bnet email, but don't quote me on that.
This is correct. Hopefully Blizzard is upgrading the forums to the new battle.net forums that Starcraft 2 uses. Diablo 3 is still on the 10+ year old battle.net forums.
Step one: Have Diablo 2 / LoD
Step two: Create a battle.net account "with-in" Diablo 2
Step three: Go to the official forums and type in your Diablo 2 Battle.net account and password and be sure to select the "realm" (example: US East) that you made your account with in Diablo 2.
Step four: Enter your Diablo 2 CD key.