-How to import the asset into my scene.
    1, using 'link' or 'append' the group''.
    2, open blend file in second blender instance, select parent object and its respective children,simply copy and paste ,there you have it! 

*this method only works within the instances of the same version of  blender . and it's much straight forward than the link and append.

-How to rotate and adjust the branch of the light fixture in blender?
select the branch, then rotate over local Z axis by press key 'R'+'Z'+Z'.this will switch the rotating axis to local Z axis.