Custom X509CertificateValidator

Hello All, I am trying to create a custom SSL certificate validator for a WCF service and for some reason, I cannot get the validation code to run.  Can someone please help me?  I am guessing it has something to do with my config file for the service but cannot figure it out.
My service config look like this:
  <system.serviceModel>
    <behaviors>
      <serviceBehaviors>
        <behavior name="myApp.ServiceBehavior">
          <serviceMetadata httpGetEnabled="true"/>
          <serviceDebug includeExceptionDetailInFaults="true" />
          <serviceCredentials>
            <clientCertificate>
  ...