Solution: If open, close the ‘Keychain Access’ app, and drag the certificate onto the app. It will load right up.

Note: DER encoded certificates will not load. Change encoding to Base64.

This can be an incredibly aggravating issue as these certs are pervasive in the intranets of organizations.

My issue involved connecting a number of Macs to Office Communications Server with the ‘Messenger for Mac’ client

Caveat: This assumes you already have the X509Anchor ring added into ‘Keychain Access’.

To do that, open ‘Keychain Access’; Go to ‘File’ –> ‘Add Keychain’; navigate to ‘/System/Library/Keychains/’ choose ‘X509Anchors’.