You dont need an app. If you have the p1102w, which is airprint compatible, you are all set, once you get it set up.
First, set the printer up, and get it to recognize your wireless network. If you are using a scure network, you will need the whole password routine....
Once you get the printer on your wifi network, using the control panel on the printer, check for a firmwarre update from hp. Most of the airprint printers need this update before they will work.
Once you have the firmware updated, from any app or screen that supports printing ( like safari). Touch the print button. In safari, it is the little box with the arrow just to the left of the address bar at the top of the screen.
One of the choices is print, and if the pad and printer are on the same wifi network, it will find the printer, and print.
The trick is to get it set up and updated before you try and get frustrated.