Ok, I got your PM. You explained that you wanted to boolean a cylinder to make a jack-hole for a plug on the front face. On your own, you discovered that this only works on my models and not yours. Here's why:

In the first image, we see the front face of your model. As you can see, the corner of the face is shaded black in one corner. This is do to one of two things: The normals are somehow messed up (normals have to do with how the model shades) or a vertice and/or edge is out of wack due to a modification on the geometry in the form of vertice pushing or similar action.

Basically in your case, you probably selected a vertice and accidentaly moved it somewhere it shouldn't be moved to and your face normals went wacky.

In the second image, I selected a face and yanked it out so I could see how you created a portion of your box. The red arrows point to extra faces that were created during your process of forming the object. I'm not entirely sure how these came to be, but I explained how to model it my way if you care to use my method next time around.

In the third image, we see extra edges, which instantly means there are extra vertices. In a previous post, I explained how to notice these extra vertices and edges and the way to delete them.

All three of these things contribute to your booleans not working. You must keep your geometry simple and know how your going to model your object before you model it sometimes.

Important: With almost every model, especialy inorganic models (models that are squarish), you should be able to select a face, move it out or in, and see the faces surrounding it move with it. This instantly tells you if the face you have selected is connected properly to the other faces. If the surrounding faces do not move with the face you are moving, they are not connected correct due to a modeling problem and should probably be fixed.

Feel free to PM me again or post another message if you need any more help. I live to serve.