Downloaded assets come up as empty objects!

Hey I was making a fps on Unity and I downloaded the files from unity and the objects are showing up as a empty object or something like that when I move them to the scene. They just show up as a cube that I can't resize and when I deselect it it disappears. Is it because I have windows XP or because it downloaded wrong. Also when I imported the assets an error message showed up saying I don't have Maya. Do I need Maya or is it my computer or is it something wrong in the download. Please Help.

The error appeared because they are Maya files that you downloaded, and Unity requires you have Maya installed so that Unity can convert them into a usable format.

They aren't appearing in the game at all because Unity has no way of reading the data until it has been exported from Maya.