I am not a software guy. I would think they would have given you instructions to at least get you printing. You first need to get the printer to communicate with the program, this involves setting up speed (baud rate) and such like with the old modems (again I am not a software guy) If they did not give you any settings you could try here [
www.reprap.org] as they use the Marlin firmware.
I would make one suggestion, dump Printrun and download Repetier Host instead ( [
www.repetier.com] ). It will doe everything Printrun does and more, it's easier to use, has a much better interface and gives you much better visual feedback (and yes, it's Mac compatible) and it will work with any printer that Printrun will.
Once you get them talking you can use the manual controls of either program to test the movement (X,Y & Z) to see if everything is working. Then load up a simple gcode and see if it prints. You may need to do calibration but again I am not a software guy so that's beyond me.
Good luck.