Also, they are put into a per-user directory, even though these are system wide files that belong in a system wide (public) directory. When you install as as admin user, but run as an underprivileged user (like everyone should) the files get put in the wrong place. Rather than putting these files into the Documents directory of the current user running the install.

HOWEVER, I would STRONGLY encourage you to consider the correct placement for system wide files to be: I ran mtllib.exe as my local non-admin user, and it copied the files to my own user's Documents directory.