Make sure you're using the latest and greatest. The most current version for Macintosh® is EndNote X7.5
Get the latest update to X7 for Cite While You Write for Microsoft® Word 2016 on Mac and other improvements. Details
(released 2 February 2016)
In EndNote X7, under the "EndNote" menu, choose "Check for Updates".
If you are still on an older version of X7 or want to install the X7.5 update manually, read this Knowledge Base article.