Depending on what kind of rotation value you wanna use, it can be a float. or a vector.
for example, you can use vector to grab all rotation axes (X,Y,Z).
then in the level blueprint, grab those values from your sun asset and assign it to the material collection parameter you created.
Does that answer you question? 
If not, maybe try to follow the tutorial on the page, the concept of the workflow came from there
Let me know afterwards if it is still not clear, i’ll try if i can still help you out.