The situation is the following:

Your CD key is already registered to a Battle.Net account, but you can't remember the account name
Downloading the installer does not help you a lot, since you need the registered 26 number key, which can only be obtained through your old account.

Therefore you will either need to buy a new version of LoD or try to remember the account name.

Edit: Another simple method would be to ask a friend for the LoD CD, since you only need it for the installation.

This post was edited by iDGames on Jun 13 2011 08:45am