Mulifit is written for the IDL Virtual Machine and should run on any platform the virtual machine is available for.
Multifit/Polydefix are now free, open source, GPL software. You can get the full source code and executables at the Multifit/Polydefix homepage.
At this point, you should have obtained several SAV files,
All are IDL virtual machine files.
Under windows or mac OS
On a unix of linux machine