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.
Also 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 to Mac OS X, you can also use Disk Utility to convert it to the Apple Mac OS X disk image file format (*.dmg).