For many years I have used the shareware music notation program http://www.myriad-online.com/en/index.h ... Assistant
. While I am sure it is not as powerful as professional music notation programs it is only $20. It will open a variety of formats in notation and can easily tranpose by simple commands.
BTW, its native format carries a .mus extension, but I am not sure that this is the same format as you are asking about. A free demos in available, which has some limitations, but should allow you to see if it will open the files you want to work with.