The old RCM is no longer being supported, and you must get the new version of the add-in from the Microsoft Office Store.

Here are instructions on how to do it.
 

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If that does not work:

1. CREATE your Word document and save it
2. SIGN OUT of MicroSoft Word (instructions attached)
3. Go to this page of the Microsoft office store
4. Click "open in Word" --> Open Link
5. Re-open your document
6. On top, go to INSERT --> Click on My add-ins --> ADD
7. RefWorks Citation Manager should open on the right

Unfortunately, you will need to do all of this every time you open your Word documents.