You can create reports from the reports manager. To do this, select the button at the top of the reports list. This will open the Report Wizard where you will be able to specify all necessary parameters for the new report:

 

The order that you select the scenarios will determine which scenario is the 'Base' scenario and which one is the 'Comparison' scenario. The first scenario that you select will become the base scenario and the second scenario will become the comparison scenario.