The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
From Andrew Wayman:
Athletic heart monitors have existed for several years now, but it wasn't until relatively recently that the technology behind them, and the development of heart monitor training techniques came together to make training with a monitor both simple and effective for the average runner. While many runners own heart monitors, often they may not be using the devices to their full potential. Other runners do not own a heart monitor and are unaware of the benefits of training with one. Calculating training zones allows you to customize your workout to your heart and current fitness level. Using a heart monitor without tailoring your workout to your own personal training zones essentially eliminates the benefits of heart monitor training. The Heart Rate Zone app from Runners Ally will help you find your custom heart rate zones, so that you can get the most out of your workouts.