Unlike PCs, Macintoshes don’t automatically trust certificate authorities that are tied to Active Directory. If you have deployed Lync Server with internally generated certificates, they will need to import them into a Keychain on the Macintosh. This is a relatively simple process and can be completed with the following steps:
1. Open Finder.
2. Expand Applications and then Utilities.
3. Drag the Root Certificate Authority certificate to Keychain.
4. Select the System Keychain and click Add.
5. When prompted, type a name and password with local administrator rights and click OK.