First of all, get a 3.1 iso, it has an important bugfix which helps
resolving boot issues from usb.

The guide on the French page is needlessly complicated, here's a simplier
solution:

1) Get a FAT32 formatted USB stick, best if you format it under the OS of
your choice

2) Get a MorphOS 3.1 iso and copy it to the stick. Mount the iso file
(easy on OSX, MorphOS, and Linux, under Windows - use google, there's a free
utility from Microsoft that does that) and copy the boot.img file from
mac_ppc32 directory on the iso to the USB stick

3) Plug the stick into your Mac, power the Mac on and get into OF (Cmd + Alt
+ O + F). Make sure to plug it directly into onboard USB, don't use hubs,
etc, this simplifies step 4. Make sure you do NOT have any other USB
disks/sticks plugged in, you'll only waste time trying to boot from the
wrong device.

4) Find out the USB path in of to your stick!
type:

dev .aliases properties

This will give you a list of root USB devices (hubs) found. Usually they're
named usb0, usb1, ...

type:

dev usb0 ls
dev usb1 ls ...

until you see something like disk@1 in the list - you've got your path now!

5) Boot MorphOS 3.1

type (assuming the disk is at usb0 and is named disk@1):

boot usb0/disk@1:,\boot.img bi umsd0:mos.iso

You should see the boot logo in a few seconds. If nothing happens after
loading, try replugging the usb stick or try a different stick.