Hi Keith
Insert > Webassist > CSSFormBuilder > Client Validation
Select the Form > 'Next'
Type lines to precede an succede the error message dialogue > 'Next'
Select the Form element to validate (i.e. PostCode) and Validation Type (i.e. Zip Code) - Click the + button
Select some or all of allowed postcode types, and additional configuration options - > OK
(Add more validation types if required)
Click 'Finish'