Disclaimer: These instructions should work for most iPhones/iPads but there may be slight variations on some devices.
Download Wherigo from the Apple Store.
Click on the link in the geocache page to get to the Wherigo cartridge page. On the left hand side, click the "Download" button.
From the dropdown box, select "Pocket PC Device" then click "Download Now".
Select the desired cartridge to load.
Click play to start the game.