To create diffusers, go to
DM HVAC 
Diffusers Diffuser Project Schedule
The Diffuser Project Schedule dialog box will appear as shown below.
Press the New button. The New dialog box will appear as shown below.
In the New dialog box, type "S-1" as the New Diffuser Callout and press OK (The dash is important when Design Master HVAC determines whether a diffuser is appropriate for the needed CFM). The diffuser is now created in the database, and the next step is to define the diffuser. In order to facilitate this next step, Design Master HVAC will automatically take you to the Diffuser Project Schedule dialog box for the diffuser S-1, and you may now enter the diffuser specifications.
Set the diffuser specifications for S-1 to match those shown below. (The specifications that you will need to change are highlighted in yellow in the screen shot below. They will not be highlighted on your dialog box. The blue highlighted area is set by the previous step, typing S-1 into the New dialog box.) Once the specifications match, press Save.
Note that the Block Number: and Block Type: and the block itself have changed. Changing the block will automatically change the description and block name. To change the block, press Select Block, choose the appropriate block from the Select Diffuser Block dialog box, and press OK.
Next, use the above process to create diffusers S-2, S-3, and S-4 with the specifications shown in the dialog boxes below. Press New, type the appropriate callout as highlighted in the screen shots below in blue for the New Diffuser Callout, and press OK. Set the specifications to match those shown below and press Save.
Finally, you will define two return diffusers using the same process. Press New, type "R-1" for the New Diffuser Callout, and press OK. Set the specifications to match those shown below and press Save.
Press New, type "R-2" for the New Diffuser Callout, and press OK. Set the specifications to match those shown below and press Save.
Now you have defined diffusers in the project schedule, and are ready to insert them on the drawing.