Wherever you put them. What settings did you use when you imported them?
Oh, that's right you didn't pay attention ;-)
So the odds are then that they are inside Aperture's "library," which is probably called "Aperture Library" and in your Pictures folder. You won't see them in the Finder, unless you right click on the Aperture library and select "show package contents," at which time you can root around there. My recommendation is to NOT touch anything, though, unless you want to screw things up.
If you don't like this, it is very easy to move the files around. You just select the project(s) within Aperture, and choose File->Relocate Masters for Project. Then pick where you want them to go. Aperture's file management runs circles around most anything else in this respect, because you can put them wherever you want, and it's trivially easy to relocate them. Pick a target location and Aperture will move them.
In the future, you can choose where the files go upon import. In the import pane, there is a box that says "Store Files:" and it probably says "Aperture Library" in your case. Change this to a specific location that you've defined. You can have it copy to a backup location at the same time, if you want.
Do have a read of the importing and file management sections of the Aperture documentation… it will clear this up.