Can't see install disc for Linux Mint in VirtualBox
From your question it sounds like you are downloading the ISO file and burning it to a CD or DVD. This isn't necessary.

Download the ISO file from the Mint website.

Then within VirtualBox create a new VM entering the various properties like memory size,  hard drive, etc.

Once the VM is created, open up the Settings for the VM, go to the Storage tab and select the Empty CD drive. Click the light CD image on the right hand side and select "Choose a virtual CD/DVD disk file...". Then locate and select the ISO image you downloaded from the Mint website.



Close the Settings dialog and Start the VM. The installer should load and run to start your install.