How to fix "A reference to 'Audio Precision AP2700 3.00 Object Library' could not be added"

I am having some dll related problems with C# in visual studio 2013. I can get this AP Hardware demo to run, but I cannot get the dll imported into my own project, or re-imported into the demo for that matter. I think I am doing something wrong from a visual studio standpoint.

I am using the AP2700 demo found here: http://ift.tt/175LgrE It runs fine out of the box through visual studio, but I cannot figure out how the "AP" reference is added in the example project. The "Audio Precision AP2700 3.00 Object Library" is checked in the COM references, but when I remove it and try to add it again, I get an error: "A reference to 'Audio Precision AP2700 3.00 Object Library' could not be added."

Here's a picture of the example project, works fine

Here's a picture of removing the dll then trying to re-add the dll