make sure that the check box has a checked value.
edit the Server Validation Server behavior, select the Required / not blank validation type.
for the server variable, click the lightning bolt and select the checkbox, then click the plus button to add it to the list of validations.
in design view, type the error message next to the checkbox, and select it.
go to webassist -> Forms -> Validation -> Validation Show If
from the list of validations, select the required validation for the checkbox, then click the plus button to add it to the failed list and click ok.