This last step is assuming you have the Deskjet 3520 e-All-in-One Printer. Last step Connect the USB cable, download and install the HP Deskjet Full Feature Software and Drivers - Mac OS X 10.6, OS X 10.7, OS X 10.8.
This download includes the latest HP printing and scanning software for OS X Mountian Lion, OS X Lion and Mac OS X v10.6 Snow Leopard.