No, it doesn't matter whether you're in admin mode or not - the installer for the addon makes the choice about whether it is installed for all users or not. You may just have to wait until I support it.
An x64 version is coming, I hope - I don't personally have access to an x64 version of Windows Media Center, so it's taking a bit longer.
Yes, MC Menu Mender replaces MC Menu Customiser. Thanks!