Installing Addons

Ok, so I got around the administrator on the macs we got at school.
Its a MacBook 10.6 Snow Leopard.
I was wondering how to install addons on this mac.

When you mean addons, do you mean garrysmod addons?
Also I think you will get in trouble if you install games on school computers. Sorry for not helping, but I think it would be good advice not to install gmod on school computers.