You could burn the .iso file to a DVD and have it ready in the drive... that is what VirtualBox is expecting. But to save DVD's, you can use the .iso file directly, but you have to tell VirtualBox where it is. Basically, you are creating a virtual DVD drive so VirtualBox will boot the .iso properly. After you finish the installation of Linux Mint, you will need to go back in the settings and delete the virtual DVD drive or else it will continue to boot on the .iso file. Here is a quick one-minute YouTube video that shows how to make it work.