If you just need to add a Floppy Disk into the disk controller, this is how to do it:
Click on the Floppy Controller. An icon of a floppy with a green plus sign should come up on the left of your selection. Click on this small icon.
A dialog should now come up:
Select "Choose Disk"
The file selection box will come up---at this point, choose your .img
file from the file selection box.
From this point you should be able to boot the virtual machine from the floppy disk and test your bootloader.