I also ran in to the same issue. When I copied the CD contents to a network location, there was a setup.ini file included. I created the MST and everything worked fine. Then I created an admin install point in another directory. Once I moved the MST to that directory and tried to modify the package again in teh new directory, I got this error. It worked fine from the CD folder. I don't know why this is an issue, because I didn't have this problem with the Acrobat 7 tuner.
Anyway, I just copied the setup.ini from the CD into my admin install point to further edit the MST.