When they say "OS-Extender" in
the Readme, what they mean is a kernel such as DoorsOS or TEOS or Universal
OS. The readme was written by people who speak French as their first
language, so it can be confusing. All three of these OSes are included
with the latest version of SMA, so you already have them on your computer.
By the way, shouldn't you be asking these questions somewhere else? This
is a Programming questions mailing list, not a "I want help with my games"
mailing list. There is a forum on the Time To Team website (www.ti-fr.org/T3) that is just for questions
about SMA and other games, and there are other places you can go to get help
running games. Anyway, good luck with SMA (you'll need it!)