Are you using windows maybe 7 or 8?
I think the problem is that you are try to sync a library vice a folder. For Windodows help:
Libraries are collections where you can get to all your documents, music, pictures, and other files in one single place. In some ways, a library works like a folder: you can use it to browse and sort files. But unlike a folder, a library gathers files that are stored in several locations. This is a subtle, but important, difference. Libraries don't actually store your items. They pull from folders that contain your items, and let you open and arrange the items in different ways. For example, if you have music files in folders on your PC and on an external drive, you can get all of your music files from the Music library.
You have to either sync from a photo program or from a plain folder, not a library folder.