Double check that you have matching name for the morphos.iso on the command line you write and on the USB stick. "morphos.iso" vs "morphos-3.10.iso" etc.
I'm quite sure that's the case. The guide says "morphos.iso", but the ISO file has different name. Hence, MorphOS started booting, but the required medium just "isn't there".
Koszer is correct! I was typing morphos.iso instead of morphos-3.10.iso
I'm at work now but I'll give it another go when I get home.