Was my mustang I supposed to come with a dvd with the recording software on it because mine didn't.
Did you buy it new or used? If you bought it new, then I would contact the store you bought it from. They may refer you to Fender.

my mustang III did come with a dvd with a link to
the fender website from where i could download the recording software.
it was not on the dvd itself.

i assume your mustang I did come with a serial code or something
to unlock the software. if that's the case i believe you can just go to
fuse.fender.com (am i allowed to link to there from here?)
make an account and download all the software you need.

if you don't have a serial code for the software then contact a web administrator
at that same website and ask him for help or contact the store you bought your amp from as suggested earlier.

hope i could help.