Before configuring your Web Publishing Rules in TMG 2010, the certificate that will be used on your Lync Server rules must be installed correctly with the private key. Instructions for installing this certificate can typically be obtained from the provider of the certificate.
If your Public Certificate Authority provides a package of certificates, it is recommended to install all certificates they provide. This can include root and intermediate certificates. If these certificates are not installed on your servers, it can often cause issues with external clients.