What year do you have. If it is an '09, you'll need to do a little work as described above to install the cover.
Here's my DIY on how to do that and the parts needed. If you have an '09, you DON'T have to buy the whole side panels. You can do it with about $80 worth of parts from Courtesyparts.com.
DIY: Steps to add rear tonneau mounts for those without