When you create a Smart Album, Aperture automatically populates it with images that match your Smart Settings criteria (similar to the Filter HUD). You cannot manually add images to it.
However, if you drag an image from a project to a Smart Album, you'll see a green plus sign (+) next to the mouse pointer that normally indicates that a file will be added to the list. In reality, Aperture won't add the file; the plus cursor appears in error.
This issue is resolved in Aperture 1.5.1