There's some information on atlatl energy
here. Using that with the bow supplement to FF&S from the World Tamer's Handbook yields the following:
Heavy dart: Dam -1, Rng 30 (27), Pen Nil
Light dart: Dam -1, Rng 20 (24), Pen Nil
The heavy dart could probably be upped to Dam 1, Pen 1-Nil based on the higher momentum of a heavy (relatively) dart compared to an arrow. Even though atlatls are able to take down deer (it's legal for deer hunting in Alabama), I don't think I'd raise the damage higher.
I'd say an atlatl is Rld 2. Having done both archery and atlatl, I can definitely pull, string, and draw an arrow quicker than I can load an atlatl.
There is no required Str. It's a fairly easy device to use once the motion is learned, and it doesn't require a high strength in the way a powerful bow does.