Making Nutella flow Smoothly
If it's okay for it to be warm, just heat it a bit in the microwave. (You could also use a double boiler, or even just carefully use a pot, but it's extra effort.)

If you don't want it to be hot, you probably want to try to incrementally mix in some other ingredient from the recipe. If you do it all at once it's difficult to get it mixed evenly, but if you add a bit at a time, it'll slowly thin and you can keep mixing well.