Files can be added to Mendeley Desktop three ways:
1. Open the Mendeley Application, then drag files from your desktop or document window over the Mendeley Desktop application
2. Click on the "Add Files" button in the toolbar of the Mendeley Desktop application to add files from a traditional upload box
3. Click on the black arrow of the "Add Files" button, select "Add Entry Manually," and input the citation information
You can also click the "Open In..." link to select Mendeley from a list of any PDF viewer applications installed on your device.