The Calculate Building Load Totals command will calculate all of the HVAC heating and cooling loads for the project. In general, the calculations happen automatically when necessary. Before printing, you have the option to calculate the loads or not. You should not typically need to run this command during the normal course of a project.
To calculate the loads, go to
Ribbon: DM HVAC->Load Calculations-> 
Calculate Building Load Totals
Pulldown Menu: DM HVAC->Load Calculations->Calculate Building Load Totals
Design Master HVAC Dialog Box
Press Yes to calculate the loads.
Press No to cancel the command and return to the drawing.