if your unit didn't come with a roof unit, you probably can't put one in without some serious modifications. therer most likely sin't any roof support for an a/c unit up there.
which leaves you with a window mount or closet mount.
personaly i don't care for a window mount. just think they don;t look very nice.
for closet mount you have to decide where your willing to lose storage space.if i were to try it, i think i would look at mounting it under the seats somewhere. or the bottom of the closet.