ok, just checked out mine, go to your control panel and on the left there will be menus for options, (go to manage garage at the top) and then you should see a link at the bottom of your first garage that says add a garage.
SE Level Members get a 1 car garage - just by being a member.
R/T Level Supporters get an upgrade to a 2 car garage.
SRT Level Supporters get an upgrade to a 5 car garage.
Look for the Upgrade link at the top right of the page, below the logo. It shows all the extras provided with each supporter level.
Both supporter levels are very minimal in cost. This is an "open" forum, of like-minded people that have varying interests in the Challenger, that allows its members to become supporters to help cover expenses - unlike other Challenger forums that are commercial entities with the primary purpose of generating income, by taking advantage of members good-hearted nature, or guilting them into becoming paying supporters.
You will never see moderators or members here encouraging or pushing members to become supporters, like you will on other Challenger forums.