Some disk image handling programs like AnyToISO or gBurner are good choice for converting .mdf images (Alcohol 120%) to DMG format which is mainly used on OS platform.
Worth mentioning is that on Mac computers you can supposedly use trick with Roxio Toast, or similar disk mounting software. Rename .mdf file extension to .dmg (or .iso) → Open Toast, go the the Copy tab → Select renamed Alcohol 120% image → Click Mount. Now you are able to view content of .mdf file.
If you rename .mdf to .iso and mount it on Mac, you can also use the Disk Utility to convert it to the Mac disk image file format (.dmg).