Square elbow and wye fittings in Design Master HVAC can have turning vanes inserted on them.
When you manually insert one of these fittings, you will be asked whether they should be inserted. If you want to change this setting for a fitting that already exists, insert the fitting again.
Turning vanes can be configured to be included on these fittings automatically in the Options.
Ribbon: DMH: Customization->Project Customization-> 
Options
Pulldown Menu: DM HVAC->Customization->Options->Edit Project List
In the Fittings section, change the Turning vanes on square elbow and wyes option.
If you change this option in the middle of a project, you will need to redraw the ductwork to have the turning vanes updated.