Searching the treats outside should begin with easier tasks if your dog hasn’t search anything before.
In the first time you can throw the treats on the side walk or in the short grass.
Later on you can throw the treats in the long grass, in the snowdrift or even hide them under some moss. Searching in the dusk or in the dark is always harder for your dog than searching in the bright daylight, because then the dog can only smell where the treats are, he can’t see them (very well at least).