To use:

2.8:
simply hit append- browse to the pack-

go to collections in the pack folder, and append the collection that says "APPEND_ALL_TREES" 

to spawn a tree, make sure you're working on your scene collection- then add a collection instance (Shift+A) -   Search for "Tree"

to Edit the Materials/seasons- zoom out in your scene and you'll find the Master Materials plane. 

feel free to tweak the Seasons, Color, translucency, etc.

-Notes:  
  use the "summer/autumn" slider to change seasons
 --The autumn color can be changed to anything matching the scene (like blue or whatevs)
-----Make sure the translucency color matches the leaf color and is slightly brighter.    or make it a completely random color- see where it takes you :)

Blender 2.7:

- works same as blender 2.8-
however, append everything on a separate layer< out and away from the scene.