There are a couple different ways for you to access and use Mendeley. The following links will lead you to each where you can decide which works best for you.
Mendeley Desktop runs natively on Windows (32 and 64bit), Mac (Intel with OS X 10.5 or later) and Linux (32 and 64bit).
Mendeley is a free reference manager and academic social network that can help you organize your research, collaborate with others online, and discover the latest research in your field. Mendeley is a great organization tool for those who have accumulated a large number of PDF files during the research process. Using Mendeley, you can extract metadata from your PDFs which will allow you to index and organize your collection for easy searching and citing. Mendeley is a desktop application but also has an online component so you'll need to download the software onto your computer in order to get started. Mendeley also offers users a mobile option called Mendeley - Reference Manager (Lite) for iPhone, iPod Touch, and iPad.
(from Mendeley's Features page)
This guide is based in part on a guide by Marie Sciangula at the Teaching, Learning, and Technology Center (TLTC) at Purchase College, SUNY.