In the Entity Generator, Entity Generation tab, dt is the amount of time the generator has to wait before it generates the next entity. So it is the delta time to next entity. Or the inter-generation time.
If I understand correctly, you want 60% entities to go down path 1 and 40% to go down path 2. Assuming entity.Route is what is deciding the route later on in your model, you can draw a uniform random number between 0 and 1. If the value is less than or equal to 0.6, then set entity.Route = 1. Else, set entity.Route = 2.