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 Brian Semiglia:
Space Defenders is a real-time parking finder that uses the power of Twitter to help you instantly seek-out and share parking spaces. The parking spaces you find are generated by other space defenders. More users means more spaces to find, so tell a friend. Enlist in the League of Space Defenders today!// DUTIES /////////////////////////////////////////////// Find public, metered, and residential parking spaces SECONDS after they become available. Find and share parking spaces anywhere on Planet Earth. Save gas and time. No need to create an account. Space Defenders harnesses the power of your existing Twitter account. Just sign in and you're ready to start defending space. And remember, you must be the change you wish to see in the universe.// AUXILIARY NOTES //////////////////////////////////// An existing Twitter account is necessary to enlist in the League of Space Defenders. Create one for free at twitter.com/signup Reporting an available space sends out a tweet from your account showing the street address of the available spot. [i.e. '64 Rainbow Rd. #PublicParking'] Location must be enabled for your account. Enabling location for your account only adds location to tweets broadcasted with Space Defenders. Tweets made with other applications will not be affected. Space Defenders is currently only available for Planet Earth.