Q: How to add a URL to a LaTeX bibtex file?

To use url with a plain bibliography style you can use this format:

@misc{Doe:2009:Online,
author = {author},
title = {Title of Citation},
year = {2010 (accessed December 7, 2014)}, 
howpublished = "\url{http://www.myurl.com}"}

Additionally you need to add the url package

\usepackage{url}

If you want to use the url attribute in you need to use natbib because standard bibstyles (such as plain) will not typeset the url key contents of the individual entries; it is required to use one of natbib’s own entries, e.g. plainnat.

For example

@ONLINE{Doe:2009:Online,
author = {Doe, Ringo},
title = {This is a test entry of type {@ONLINE}},
month = jun,
year = {2009},
url = {http://www.test.org/doe/}
}

and the document

\documentclass{article}

\usepackage{biblatex}
\bibliography{test.bib}

\title{BibTeX Website citatations with the \textsf{biblatex}~package}
\date{}

\begin{document}

\maketitle
\nocite{Doe:2009:Online}
\printbibliography

\end{document}

Source: http://nschloe.blogspot.ro/2009/06/bibtex-how-to-cite-website_21.html