If you have an iPhoto library already on your disk, you can add it to your list by clicking the "Add Library" button and selecting the library's folder on your hard drive. You can also add a folder by dragging it from the Finder into the list of libraries. If you want to know where one of the libraries in your list is located, click the Reveal in Finder button to show the selected library in the Finder.

The folder you choose to add must be a library folder that iPhoto has worked with already. You cannot treat a simple folder full of pictures as a photo library and have them show up in iPhoto. If you have pictures already in a folder that you want to work with in iPhoto, you must first either create a new library or open up an existing one, and then import those pictures from that folder into iPhoto.

See also:

Creating a new library

Removing a library

Duplicating a library

Moving a library

Renaming a library