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.
Do The Roar will allow anyone to annoy Shrek and cause him to bellow out his enormous Ogre roar. Use Butterpants to help you annoy Shrek - just keep punching the button or shaking your phone, and the little boy will badger the big ogre. Finally, Shrek will let loose with his famous roar. Be careful, you may need to clean your phone afterward.