When your certificate is issued you will receive 3 certificates:
Yourdomain.crt
ComodoClass3SecurityServicesCA.crt
GTECyberTrustRootCA.crt

• Add the ComodoClass3SecurityServicesCA.crt certificate as Trusted Roots:

• Then attach each certificate in turn to your website's Key Pair in the following order. At this point your Key Pair will be black.:
  Yourdomain.crt
  ComodoClass3SecurityServicesCA.crt
  GTECyberTrustRootCA.crt

• Your Key Pair will now turn green.
  Stop and Start the server, your site can now be found using the https entry