1. If the "Add Printer" is "greyed out" (like it was for me when I upgraded to Lubuntu 15.04) and you cannot select it, then look to see if the wifi printer shows up as a wifi network under the wifi network list (you can see the list by clicking on the wifi strength bars at the lower right of your screen).
2. At this point I selected disconnect, then tried to connect to the wifi network printer (it didn't let me connect, it said I needed a password).
3. When I went back to the "System-Tools-->Printers" window, the "Add Printer" button was now not greyed out, and my wifi printer showed up as a printer to add.