If you develop a custom validator, you need to also create a custom service module controller. As an example, create a fille called DemoServiceModuleController.xml with the following structure:

<ServiceModuleController validator="MyCustomServiceModuleValidator" globalPanel="GlobalPanel" optionalPanel="OptionalPanel" submitControl="imgBtnSubmit">
<Control required="true" name="txtRequiredField1" message="Please enter the Required field 1 value"></Control>
  <Control required="false" name="txtOptionalField1"></Control>
<Control required="true" name="txtRequiredField2" message="Please enter the Required field 2 value"></Control>
</ServiceModuleController>