Check the box in front of the Cite While You Write option
Click next until the Customizer is finished.
Then open Word and check the Ribbon for the EndNote tab
If you are not seeing the EndNote tab, follow these steps:
Close Word and EndNote.
Open Finder>Applications>EndNote X9>Cite While You Write>EndNote CWYW Word 2016.bundle
Copy the file "EndNote CWYW Word 2016.bundle" by highlighting the file and going to Edit>Copy
Open Finder>Go>Computer>Macintosh Hard Drive. Then follow the path Library>Application Support>Microsoft>Office365>User Content>Startup>Word and paste the file "EndNote CWYW Word 2016.bundle" by going to Edit > Paste
Start Word, go to Word>Preferences and click on File Locations.
If the Startup folder is set to any location, highlight Startup and click on Reset
Click OK and close Word
Start EndNote and then start Word.
Thomson Reuters Scientific have some instructions for solving problems with the EndNote toolbar and various versions of Word available here.