Have you actualyl checked the relevant settings on the camera itself? That aside, such a flat scene will give barely any DOF, so it's probably not going to work, either way. You may need to create the illusion using the Lens Blur effect or similar...
Select the camera layer and press 'aa' to reveal the camera properties.
Select the 3-D layers and press 'uu' to reveal the modified properties.
Enable 2 views so you can see the separation of the layers (top will work).
When you get that far you should see something like this:
Everything works as it should. Because you did not reveal any of the settings I can only assume that the Depth Of Field switch is not on, or the aperture is too small for the distance between the layers. It works just like a real camera except that aperture is the diameter, not the F-stop. An aperture setting of F 2.8 gives you a big diameter iris and a shallow depth of field while F 16 dives you a small diameter aperture and a deep depth of field. Set the aperture setting for AE's camera to 10 pixels and you will have to move the layers a very long way apart to notice any blurring.
Two views are also very helpful because you get a view of the focus distance so you can line it up on the layer you want in focus easily.
You will find more information and instructions if you use the search help field in the top right corner of After Effects and look for "depth of field." There is also some good information in the User Guide. You can find a link on the landing page of this forum and on the After Effects product page.