You can find the latest update at Update for Lync for Mac 2011. If you use a version that's earlier than version 14.0.6, you may have problems when you sign in to Skype for Business Online because earlier versions have issues that prevent authentication to Skype for Business Online. To connect Lync for Mac 2011 to Skype for Business Online in Office 365, you must have Lync for Mac version 14.0.6 or a later version installed.