I imagine that the camera's scene & Auto modes will only work with a CLS (creative lighting system) iTTL flash like the internal one, or the SB-400, 600, 800 or 900.
To use an external flash in its auto mode, you'll need to have your camera in A (aperture priority) or M (Manual) mode. Then you'll know what aperture is in use, and will be able to set that aperture on the flash.
Some speedlights have an AA mode, where the camera communicates the aperture to the flashgun. I don't know if the SB-24 is one of those.