you would need to order it on the order page:
http://rentals.nuclearfallout.net/rent. ... tent=order
it will show up in your control panel after you start the subscription.
As for unloading amxmod, you can do this:
meta unload amx
or if that doesnt unload, meta force_unload amx
thats how it would show up in cal.cfg if you are going to place it in there.
if you want to use rcon to unload them, just simply put rcon before it
its best to turn off metamod before you are going to have your match, which you can do in server control tab of your control panel. uncheck the box and hit set, then restart the server. You can do this anytime before your match, then at the match, use rcon.