Verify that the new certificate appears and marked with a small blue point with a white plus sign in it. For information about these clients when used with other servers, see the Client comparison tables for Lync.

Now, in the Keychain Access application, navigate to System under Key chains and select Certificates under Category on the left pane. Skype for Business Server 2015 and Skype for Business Online support the following previously released clients: Lync 2013, Lync 2010, Lync 2010 Mobile, Lync Phone Edition, and Lync 2010 Attendant.

You will be asked to insert your password again, insert it and click “Update Settings”.

Then click “Always Trust” on the popup window.

Expand Trust by clicking the small arrow next to it and verify Always Trust is marked.

The Keychain Access Insert your password and click “Modify Keychain”.

You will be asked to put in your password to approve the Keychain modification,.

The following message will popup, change the Keychain to “System” and click ‘Add’.

Locate the certificate file (.CER) you have received from your system administrator and double click it.

Open the Keychain Access application, you can find it in the Launchpad or find it with Spotlight.

The following guide was written by a colleague of mine, Lior Gilboa and it explain how to install a root CA certificate on a MAC client:

Skype for Business 20 clients can be seen in Microsofts 'Client comparison tables for Skype for Business Server 2015' article.

When the focus belongs on you and not your room, you can blur or customize your background during a video call in Skype. Become more productive faster with the easy-to-use user interface. Exit Skype For Business on Mac if it’s running Drag the Skype For Business on Mac application to the Trash Remove the existing Skype preferences if these hidden files exist. Those root CA certificates are required in order to sign-in into Lync rather it’s on the internal or external network. The Mac client has tended to be the poor cousin, at least in terms of features support, of the various published Skype for Business clients. Skype for Business offers new and improved features that enable businesses to: Increase adoption with the Skype-inspired client. Recently we got some support request tickets regarding MAC clients which weren’t able to sign-in into Lync because they were missing the necessary root CA certificates.

Skype for business mac client