A number of people have noticed that the “out-of-the-box” installation of Mendeley differs slightly from APA6 in the way it formats bibliographies. Bibliographies are single spaced instead of double spaced, and a period is incorrectly inserted after the DOI.
To fix this, you first download the latest Citation Style Language (csl) file for APA6 from here: http://www.zotero.org/styles (Mendeley uses csl files the same way Zotero does, so the file works in both applications).
Next, put the file in the following directory: C:\Program Files\Mendeley Desktop\citationStyles-1.0. If the computer asks you if you want to replace the existing file, say “yes.” Start up Mendeley, choose View|Citation Style and select APA 6th edition.
Finally, to fix the double space issue, click once anywhere on the list of references to highlight the entire list. Then find Word’s menu for “Line and Paragraph Spacing” and change it from 1.0 to 2.0.
If you have trouble with this, contact firstname.lastname@example.org