The Print3D toolbox is located in Tool Shelf on the right-hand side of the 3D View window. It has 3 main sections, as seen in the following screenshot. The first, on the left, has buttons for the checks you can perform and statistics you can get about the object. The second, seen in the center, has tools for cleanup and exporting the model. The third, shown on the right, gives you a readout of what has been found and buttons to display the affected polygons.