If you've found that your files aren't being uploaded and appearing on your Mendeley Web library at http://www.mendeley.com/library/, first check that Mendeley Desktop has been configured to upload your files.
You can configure which files get uploaded by first selecting All Documents in the collections list to the left. From here, click on "Edit Settings" above the centre column and configure this as best suits you. Now, click on "Apply and Sync" and wait for this sync to finish.
If you find that your PDF files are still not appearing in your library, please write to us, letting us know an example of a reference that is not being uploaded, and the username (email address) you are using to log into Mendeley Desktop and your web library.