Once you are done configuring the parameters associated to your custom FlexReport, click the "Save & Run Report" button in the upper right corner of the page:

Follow the instructions in the pop-up screens that follow by clicking "Save & Run Report":

You will also have the option of exporting your report to .csv as well:

Your FlexReport is now saved and will appear on the main FlexReport screen.

For more information associated to FlexReport creation, see:  Create FlexReport