syrup of figs will 'move' you better than any laxative... in fact if you take plenty you will not need a diet!!! (The last comment was purely tongue in cheek & not to be tried)
Atkins recommends isphagula husk. You could also use fibogel. Both these add extra fibre to the diet and help. Senna will "get things going" but should be used wiuth caution and not on a regular (sorry!) basis
also drink LOADS of water and make sure you take your allowed carbs in a high fibre format. Atkins only counts digestible carb so the higher the fibre the more you get for your 20g of atkins carb