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 ATS Heritage:
Discover the National Portrait Gallery, London with this exciting new App designed to be your perfect companion, whether you are visiting the Gallery or browsing the content elsewhere. Videos and commentaries reveal 400 years of history, biography and fame, told through the amazing life stories of the people who have shaped British history and culture. You can also enjoy highlights of the Collection selected by theme including Kings and Queens, Science and Discovery, Fame and Celebrity, and Writers. Listen to audio commentaries on the works while viewing zoomable pictures of them and use the Gallery maps to plan your visit.This Talking Guide App will entice you to Take Another Look at this famous collection.