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 applications 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.
Open your Word document and click on the References tab in the ribbon. Notice that the Insert Citation icon now has a Mendeley logo superimposed.
Position your cursor in your document at the place where a citation is required and click on Insert Citation. 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. Notice that the Insert Citations button will change to Edit Citation. Click on the Edit Citation button and then on the citation in the Mendeley popup.
You may then make any of the changes allowed for in the popup.
Click on the pull down menu for Style in the References tab of the Word ribbon.
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.