If EndNote was installed prior to installing Office 365, its Cite While You Write add-in won't appear.  It needs to be reenabled.

1. Ensure Word is closed.
2. Run Configure EndNote.exe from either of the following version-specific locations:
   
   1. EndNote X20 is found here – C:\Program Files (x86)\EndNote 20\
   2. EndNote X21 is found here – C:\Program Files (x86)\EndNote 21\
3. In the EndNote Configuration window:
   
   1. Select Configure EndNote components, then select Next >"
   2. Check the Cite While You Write add-in for Microsoft Office checkbox, then select Next >.
   3. Select Finish.
4. Open Word and verify that EndNote shows in the Ribbon.