FYI if you have the same problem: this works:

- Download the ISO, and burn it both on CD and a USB stick.
- Start up the system from the CD (since it won't see the USB from bios), but do insert the USB.
- Now, when linux starts you'll notice it finds the USB and starts booting from the USB.

So, you won't need to specify anything specific, just insert both CD and USB with the same (bootable) ISO and it works like a charm.