I encountered this problem and the fix was simple.
Once you have added the libBox2D project into your VS project right click on your startup project solution and select properties.
This will open a window that shows you the currently referenced projects that your start up project depends on.
Notice that the libBox2D isn't in this list... Yet.
Click on the Add New Reference but at the bottom of the references window and select your recently added libBox2D solution.
Click okay and hit build.
You should find everything in working order.