EndNote 20 for Mac

1. Open EndNote, and go to EndNote20>Customizer
2. Check the box in front of the Cite While You Write option
3. Click next until the Customizer is finished.
4. Then open Word and check the  Ribbon for the EndNote tab

Alternatively, try the following:

1. Open a Word document.
2. Go to Tools > Templates and Add-Ins.
3. You should see the below EndNote files listed under "Global Templates and Add-ins"
   
   EndNote CWYW Word 2016.dotm
   and
   EndNote CWYW Word 2016.bundle
   or
   EndNote CWYW Word 16.bundle
4. If any of these file(s) is unchecked, check it, and click OK.
5. Close and re-open 

If you are still not seeing the EndNote tab, follow these steps:

1. Close Word and EndNote.
2. Open Finder>Applications>EndNote 20>Cite While You Write>EndNote CWYW Word 2016.bundle 
3. Copy the file "EndNote CWYW Word 2016.bundle" by highlighting the file and going to Edit>Copy
4. 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
5. Start Word, go to Word>Preferences and click on File Locations.
6. If the Startup folder is set to any location, highlight Startup and click on Reset
7. Click OK and close Word
8. Start EndNote and then start Word.

Clarivate have some instructions for solving problems with the EndNote toolbar and various versions of Word available here.