The Perpendicular Connection command is used to make a perpendicular connection between a duct and one or more diffusers.
To connect diffusers to a duct with a perpendicular connection, go to
Ribbon: DM HVAC->Diffuser Connections-> 
Perpendicular Connection
Pulldown Menu: DM HVAC->Diffuser Connections->Perpendicular Connection
You will be prompted to specify the diffusers to be connected.
Select diffusers to connect:
You will then be prompted to specify the duct to which the diffusers will be connected.
Select duct or label to connect:
The diffusers will be connected to the ductwork with a perpendicular connection.