This instructable will go over the new Instructable's for iPhone app (which you can read about here). The observant among you may notice its actually being run on an iPad (bring us an iPad version Eric!!!).
First things first, you need to have a device capable of running the app such as an iPhone, iPad, or iPod touch. You then need to download the app (which can be done by clicking this link or searching www.instructables.com in the itunes store)
Now that you have downloaded the app, tap the icon on your device.