The iPad file sharing system Apple implemented is rushed, and flawed. If you still haven’t tried it, you’re basically forced to plug the iPad via USB, open iTunes, select an app that supports this feature, choose a file from your computer and copy it into the app database. This is not a sync, it’s a Mac > iPad file sharing, and it’s broken and slow. MacObserver has a great and insightful article about it.
Fortunately some developers are thinking about a solution for this, and I’ve come across this application called “Folder” in the App Store which has turned out to be pretty useful.