Inserting return ducts uses the same basic command and process as used above to insert supply ducts. To insert a return duct, go to
DM HVAC 
Duct Centerlines Insert Duct
Insert the return duct running parallel to the supply duct in the hallway. Specify the first and second points of the return duct as shown below. Note that these are similar first and second points to those used for the supply duct. The return duct should point in the same direction as the supply duct, as in Design Master HVAC all ducts should always point from the air handler to the diffusers, not necessarily in the direction of airflow.
Set the Duct Information dialog box specifications to match those shown below. (The specifications you will need to change are highlighted in yellow in the screen shot below. They will not be highlighted in your dialog box.)
Your drawing should now match the one shown below.
The last duct to insert is a return duct going into Conference 102. Use the Insert Ducts command, and set the end point of the duct just far enough into the conference room to reach the return diffusers. Your drawing should match the one shown below.
Now that both ducts and diffusers are inserted, you can insert duct to diffuser connections.