There are two (2) ways to insert a citation into Mendeley. The citation will insert where your cursor is placed when you select Insert Citation.
From Microsoft Word, go to the References tab and select Insert Citation from the Mendeley section.
In the pop-up box, search your reference library by entering search terms or author names, then select the reference(s) you want to insert and select OK.
Another method (this method works really well if you have a folder with citations that you intend to use for a certain paper), is to go to the References tab and select Insert Citation from the Mendeley section, then select Go to Mendeley in the pop-up box. From your Mendeley library, you can select the reference(s) you want to insert and then select the Cite icon in the toolbar to insert the citations into the document.