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