You will need to install the Mendeley MS Word Plugin before using Mendeley citations in your Word documents.
First make sure that all instances of any Microsoft applciations are closed.
Open Mendeley Desktop and click on Tools, Install MS Word Plugin.
The plugin will install and you should get a success message with instructions on how to use it.
Menedley inserts a new tool bar into your Word document.
Position your cursor in your document at the place where a citation is required and click on Insert or Edit Citation in the Mendeley toolbar. From the search box select your reference.
Note that only the intext citation is inserted at this stage.
Position your cursor where you want the bibliography to appear, check your citation style and click Insert Bibliography to create a bibliography.
You should not attempt to edit citations or bibliography entries in Word. If any reference information needs to be changed it should be changed in Mendeley and the bibliography reinserted.
Some editing of citations is possible. To edit a citation click on it. Then click on Insert or Edit Citation in the Mendeley toolbar.
Then click on the citation in the search box.
You may then make any of the changes allowed for in the popup.
Click on the pull down menu for styles in the Mendeley toolbar.
If the required style is not listed click on More Styles. This will open Mendeley Desktop and expose a longer list of styles. It is also possible to import new styles from this screen.