64 bit CPUs have been out for a while, but 64 bit Windows was only released in April 2005 (http://www.microsoft.com/presspass/press/2005/apr05/04-25Winx64LaunchPR.mspx). BE 10 was released in January 2005, so predates it. As a result, Backup Exec 10 (including 10d) are not supported on 64 bit Windows for the media server.

Backup Exec 10d supports 64 bit Windows for the remote client. You need to install Hotfix 1 to get the agent installation files.

As Backup Exec 11 was the first major version to be released after Windows x64, so it makes sense that it's the first to be supported for the media server.

For more information see the Backup Exec 10d Software Compatability List