Hi,
The specially priced upgrade ($35 in the US) for MIX owners is only available on the DigiStore:
http://store.digidesign.com
Look under Pro Tools Legacy Upgrades on the US store, listed on the left side. Or under Pro Tools Upgrades on the European stores. You will be able to choose either a download of Pro Tools TDM 6.4.1 or the Pro Tools TDM 6.4.1 CD.
Your MIX system does not have to be registered, and you do not need to have any previous installation of Pro Tools to use the full installers, which only work with MIX systems. You will be able to download after you order, and you will also automatically be sent the Pro Tools TDM 6.4.1 CD.
Pro Tools 6.4.1 is the last version that will work with MIX systems. For compatibility information, please see the MIX links in the Compatibility Documents section in Support:
http://www.digidesign.com/compato
Pro Tools|24 MIX, MIXplus, MIX3
Pro Tools 6.4.1 for Mac OS X 10.3
http://www.digidesign.com/compato/osx/mix/
Pro Tools 6.4.1 for Windows XP
http://www.digidesign.com/compato/xp/mix/