How do I add the URL to the Nature citation style?

I would like to have the link (possibly a PubMed one) to the bibliography in my thesis. I could not figure out how to do it using the CSL editor. Also, can one retrieve all the URLs automatically, or do I have to put them manually in all of my papers, in case I don't have them? Thanks! PS. Working with MacOSX 10.7.5, Mendeley 1.9.2 (Mac) and Texmaker 4.0.2 (Mac)