yeah me too so it's weird. How can I turn it into a DMG whilst in the Mac OS please?
You use the Disk Utility for that. First create an image of the size that you need. It basically mounts an image on your desktop that you can then copy files to. Once done, eject the disk and use the Disk Utility again to turn that into a compressed disk image. It's fairly straight forward, so you shouldn't have too much difficulty.