Mine won't use the litter tray either if it is nice outside (but if its a little bit rainy, windy or snowy I better have a clean tray handy!).
Mine needed no training at all. However, I have heard that if you mix the litter with soil from the garden and gradually increase the soil as you move the tray nearer the outside that can work.
As for the theory that cats dont poo in their own garden, it's poo. Mine leave me interesting and aromatic little nuggets which you can guarantee I dig up whilst they are fresh!!!!!!!