Be sure to set the OnServerValidate="myFunction" in the validator. If your just checking for length in a text box the RegularExpressionValidator would prob be best to use, it will generate the client side javascript to validate it for you if the browser supports it (the beauty of .Net). But webforms will always validate on the server-side also. Its just nice to catch a trip to the server if the users browser supports client-side validation and like i said asp.net will write the client side script for you if you use one of the packaged validators... if you must use the custom validator then you will need to write your client-side javascript function yourself and point the ClientValidationFunction property to this function.
Hope this helps:)
