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If the steps above did not work, close both Mendeley Desktop and MS Word > re-launch Mendeley Desktop > run the .exe file as administrator or double-click > re-launch Word > insert a citation
Download Mendeley Desktop and then install the Word Plug-In from Mendeley desktop. Before installing the Word plug-in, make sure Word is not open on your computer. Open Mendeley Desktop and select "Tools"->"Install MS Word plugin". If the word plugin is already installed, this option will be changed to "Uninstall MS Word Plugin". If that's the case, you can skip this step.
MS Word Plug-In on a Mac
On Windows MS Word the Plug-In will be under the "References" tab
Once you click on "Insert or Edit Citation" a pop-up box leads to your Mendeley desktop. Click on "Go to Mendeley"
...and choose citations to insert. Click "Cite". You can also insert multiple citations and later use the Merge feature in Mendeley to combine them.
By default the output style is in APA 6th Edition but you can choose other output styles. Information on editing a Style.
Final bibliography can be inserted and output style can be changed at any time.
Final bibliography in APA.
You can select all your citations in Mendeley desktop and drag and drop them into MS Word document. The output citation style can be changed from within Word (Choose Citation Style).