As far as I understand (I don’t use MasterTracks) the “current” (but old”) version is numbered 6.
While the distribution file of Encore 5 was either a demo version (available at several places), or a production version which requires a serial number,
for MasterTracks there was only a single version, which runs in demo mode till one enters the serial number.
Therefore, I’m surprized that you could not enter your serial number to activate the version you have. This should be possible on the “about” panel.
If that would not be true, perhaps one of the current MTP6 users still has the installation file, and could share it with you.