First you need to install the APA 6th edition style in your Zotero standalone software. Then you need to select that style in your plug-in.
To add APA 6th edition as a citation style in your Zotero standalone software:
Open Zotero (desktop version).
Click on Edit, then Preferences.
In the Preferences window, click "Cite", then "Get additional styles".
Search for "apa" and then select "American Psychological Association 6th edition".
Now open Word and insert a citation. If you are not automatically prompted to select APA 6th edition, click on ‘Document Preferences’ in the menu for your plug-in, select to from the list shown below, and click OK.
