Click the plus button to add more validations to the server validation server behavior. You will want one row for each field validated. Then you can select each row and edit the field and validation type for each field.
Hiding and showing the validation message is done as a separate step. Highlight the message you want only shown when the error occurs. Then select the server behavior "Webassist->Validations->Validation Show If". Then you can define when the message should or shouldn't be displayed based on the validation result.