The Calculate Building Load Totals command will calculate all of the HVAC heating and cooling loads for the project. Typically, you will have the option to calculate loads for commands where a calculation is needed, and will not 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 the Yes button to calculate the loads.
Press the No button to cancel the command and return to the drawing.