In order for photos to show up in the iPhoto window, they must first be imported into the iPhoto library. If you already have a folder full of photos you want to use with iPhoto, just pointing iPhoto to the folder full of pictures won't make it automatically recognize and be able to work with them. To make a library with the folder full of pictures, you should first make a new library in iPhoto Library Manager, open that library, then drag the folder of photos from the Finder into iPhoto.

See also:
Creating A New Library