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 Ymir Mobile ehf:
Mobileguide Iceland is an easy to use application designed to help you find your way around while in Iceland. It contains information on things to do and see in Iceland, both free activites as well as services provided by various companies throughout the country.The content of mobileguide is dynamic and devided into packages which are available through an in-app content store so you can choose to get only the content you feel is relevant to your trip. All content is free of charge. The application also contains a map of Iceland. The map is based on map tiles from OpenStreetMap.org and is downloadable through the content store. The map can also be accessed online but because of high roaming fees we strongly suggest you download the map while on a wifi connection.