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 United Nations Foundation:
The app makes it easy to get involved and includes some innovative (and fun) ways to learn about the work of the United Nations. An interactive photo-scramble game, "Pieces of Peace," gives users the ability to have fun while they learn by unscrambling photos taken around the world that are related to the work of the UN. The game includes ways for users to learn as they play, helping build awareness and knowledge about international issues. Integrated social media options also allow users to organically share this content with friends, brag about their photo-unscrambling prowess, and encourage them to get involved.