You can add .pdf files directly from your computer into Mendeley Desktop.
1. Click on Add Files under File in the menu bar to choose a .pdf to add to your library.
OR
2. Simply drag the .pdf file into your library.
In both cases, Mendeley will "read" the file and create a citation.
Always check the resulting citation carefully. Mendeley sometimes may be unable to provide full citation information.
Mendeley allows you to specify folders on your computer that will be "watched" for new .pdfs.
Every time you add a .pdf to a watched folder, Mendeley will automatically add that .pdf to your Mendeley library and generate a citation.
To create a watched folder, go to File on your Mendeley Desktop menu bar. Pull down the drop down menu and click on Watch Folder. Browse your computer's files and choose a folder. Click on the box next to it. That folder will now automatically add references to your Mendeley library.
Your personal Mendeley library will consist of both citations and (in Mendeley Desktop) .pdf documents. You can add these references in a variety of ways as described in the boxes on this page.
Always check your references for accuracy.
If your citation is incomplete, but you have the DOI for a work, enter it in the DOI field in the right details pane, and click on the magnifying glass icon. Mendeley will look up the missing information.
Trying different import methods can also help.
Citations can also be added manually. In Mendeley Desktop, use Add Entry Manually under File in the drop down menu. In Mendeley Web, click on Add Document.
Another way to add citations to your Mendeley library from the Web or UCI databases, is to use the Mendeley Web Importer.
To install it go to: http://www.mendeley.com/import/
Make sure your browser allows pop-ups or go to your browser preferences to allow pop-ups from Mendeley.
Drag the 
icon to your browser toolbar.
When you find a document on the Web or in a database that you want to cite, click on the tag in your toolbar, and complete the pop up window instructions to generate the citation. If you've also downloaded a .pdf, you can click under Files in the bottom of the right pane to add that .pdf to your Mendeley library.
You always need to double check the information it provides, however. Mendeley can sometimes not extract complete or correct citation information.
You can also import references from other reference managers like Endnote and Zotero.
You can find detailed instructions at the Mendeley Resource Center website:
For more general information about adding references, watch the MENDELEY VIDEO TUTORIAL: IMPORTING DOCUMENTS INTO YOUR LIBRARY
http://www.youtube.com/watch?feature=player_embedded&v=qRiAIaqdAOg
