After selecting Print3D, the Printer listed at the top of the printer list says Custom Printer... but I can not see how to add a printer that is not in the list.. 

After I select a plane to define the face of the model, I then enter the new printer's Width, Depth, & Height, and I can then Save to File, but I can not add a new custom printer for later use, nor can I access the Printer Properties button