A Word plug-in is installed automatically when you install Zotero on a PC. If a warning about macros appear in word, you have to activate macros in Word in order to see the tab Zotero.
If you still have a problem: go to Zotero and Edit > Preferences > Cite and the tab Word Processor. Install the plug-in for Microsoft Word once again, even if it says it is already installed.
If the tab Zotero still does not appear: go to Zotero Tools > Add-ons and click on Disable the Zotero Word for Windows Integration. Then close both Word and Zotero, and then enable the Zotero Word for Windows Integration. Close Zotero once again before you open both Zotero and Word. Sometimes you have to re-install the wordprocessor plug-in yet again in order for the Zotero tab to appear in Word.
If you use a Mac computer, you may have to install Zotero’s Word plug-in directly in Word. Open Word and go to the tab Tools > Templates and Add-ins, click the option for Zotero.dotm and press OK. Now Zotero’s Word plug-in should be visible.
As always: restart the program if you get problems after an installation.
