What should we give in the "Advanced Bootable Disc" tab?
You don't need to do anything special to make ubuntu ISO as bootable. It is bootable. Just use any image burner program to burn it to ISO. Burn at low speed for better results.
Please make sure you are changing the boot order from BIOS. You will have to hold down the F2 or Delete key or whatever key to enter your bios settings and make the cd drive as first bootable device, (carefully) save changes, and exit.