dmg filetypes are so-called Disk iMaGes. In the mac finder, double click on the dmg file and a virtual disk will appear on your desktop or in the left column under Devices. The bin file you wish to put in the root of the card will be in the virtual disk. When you have finally finished with the process you can dismiss the virtual disk by ejecting it. This will leave the dmg file on your machine. You may keep it or delete it as you see fit.