Once you download and install Mendeley Desktop to a computer, you can install the Microsoft Word Plugin. Open the Desktop, then click on Tools >Install MS Word Plugin. You may have to close and reopen Word for the install to be completed.
If you use OpenOffice on a Linux or Windows machine, you should be able to install the plugin by downloading and installing Mendeley Desktop and then clicking Tools >Install OpenOffice Plugin in the program window. For more help, log onto Mendeley Web, click on Search, then Support, and use keywords to search for your troubleshooting topic.
Using the Plugin
Mac and Windows
Depending on your operating system, the MS Word Plugin will appear differently. If you are using Word for Mac, you will have a floating bar or an Add-in, depending on your version of Word (Pictured: Word 2016 for Mac):
If you are using Word on Windows, the Mendeley Plugin will be integrated in the Word Ribbon. Unfortunately it closely resembles the Built-in "Insert Citation" tool in the Ribbon, so be careful to choose the "Insert Citation" with the Mendeley icon on it (Pictured: Microsoft Word 2013):