Not sure bout the exact commands, but the USB boot requires both the OF and MorphOS to cooperate with the USB stick.
You can remove the OF from the equation by putting the boot.img onto an OSX partition and then locate it their in OF.


And offcourse make sure you allways use the boot.img found in the macppc32 (?) folder, the one in the root dir of the ISO will only work for the Pegasos2.