Head on over to the Updates section of the app store, the latest version (version 1.5) should be showing up for immediate download. Otherwise download via the link.
Earn up to 3 GB of extra free space for uploading photos automatically (in 500 MB increments)
Automatically upload photos and videos using Wi-Fi or cellular data plan.
Update: it will not stack with previous photo and video uploads (http://forums.dropbox.com/topic.php?id=62359&replies=40)
Nope, it does not work if you have previously earned 3Gb from uploading via PC/Mac.