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.
Share your location with your friends and not the whole world. Marco! uses a reverse checkin - your friends send you a Marco! asking for your location, and you respond (or not) directly to that friend. Only those friends you choose can see where you are, and only when you choose. Plus, Marco! uses Facebook connect, so there's no need to refriend your friends on yet another social network.