The system is capable of calculating weight and balance of your aircraft.
Go to the Day View and open your reservation.
Click Manage passengers & Weight and Balance
Add a passenger by clicking on an empty seat. In the same way, you can edit or remove a passenger. Passengers can be shuffled around by dragging the passenger icon.
Proceed by adding cargo (those are also a part of the equation), and fill out flight details pertaining to weight. Do not forget to press Recalculate every time data is added.
Load Manifest is available by pressing the corresponding button in the bottom right of the Weight and Balance menu.