Insert Throw Arrow
To insert a diffuser throw arrow, go to
Ribbon: 
Insert Throw Arrow
Pulldown Menu:
You will be prompted to select the side of the diffuser on which the throw arrow should be inserted.
Select side of diffuser to which to add throw arrow:
A throw arrow will be inserted on the drawing.
Related Options
Show arrows on 4 way throw patterns: Sets whether throw arrows are inserted along with diffusers.