add the checkbox to your form, and set the checked value to 1.
In the server behaviors panel, double click the Server Validation server behavior to edit it.
Select the Required / Not Blank / Selection made validation type.
Click the lightning bolt next to server variable and select the Checkbox from the bindings, then click the plus button to add it to the list of validations to perform.
To show an error if the checkbox is not checked, type the error on the page and select it. On the server behaviors panel, click the plus button and select WebAssist -> Validation -> Validation Show If. select the checkbox validation as the trigger.