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 Jack Rostron:
OneTap connects you by bringing back essential functionality to your iPhone!Speed dial has been around for years, but it is a feature that is missing from the iPhone.Our phones have evolved over the years and so has how we interact with them. OneTap allows you to create "launchers" on your home screen to Phone, SMS, Email, Tweet and FaceTime your contacts!Instead of having to swipe through pages and pages of apps trying to find "Phone" or "Messages" to get in touch with the same person everyday why not just have an icon specifically for them? How about a folder for them so you can quickly get a hold of them through different means? Whatever your setup, OneTap is here to connect you.So what are you waiting for? Just two-taps is all it takes to get the app onto your device.