To insert a straight tee fitting, go to
Ribbon: DM HVAC->Ducts-> 
Straight Tee
Pulldown Menu: DM HVAC->Fittings->Straight Tee
You will be prompted to specify the duct on which a straight tee is to be inserted.
Select duct or label to insert fitting at:
A straight tee fitting will be inserted on the drawing, as shown below.
