If you want to stick with natbib, the best you can do is probably this.
Note that I kept the title field, as it is a good idea to have a little more than the author to go by.
You can use the howpublished or note field to provide the information about URL and access date (though, semantically, howpublished is probably only apt for the URL, not the retrieval date).
\documentclass{article}
\usepackage{hyperref}
\usepackage{natbib}
\begin{filecontents}{\jobname.bib}
@misc{Imager2013,
url = {http://www.zf-laser.com},
year = {2013},
author = {{Z+F Imager}},
title = {{Z+F} {Imager} web-site},
month = {09},
note = {Retrieved on 18/09/2013 from \url{http://www.zf-laser.com}},
}
\end{filecontents}
\begin{document}
\citet{Imager2013}
\bibliographystyle{apalike}
\bibliography{\jobname}
\end{document}
which gives

If you do want to use the url field, you will have to modify the apalike style quite a bit, as it is as of now unaware of any url-related features.
If you don't mind using biblatex, the following code snippet is the way to go.
biblatex offers a natbib compatibility mode (see ยง 3.7.9 of the documentation) and JabRef supports biblatex (see here).
\documentclass[american]{article}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[backend=biber,style=apa,natbib,urldate=short]{biblatex}
\usepackage{hyperref}
\DeclareLanguageMapping{american}{american-apa}
\addbibresource{\jobname.bib}
\begin{filecontents}{\jobname.bib}
@misc{Imager2013,
url = {http://www.zf-laser.com},
year = {2013},
author = {{Z+F Imager}},
title = {{Z+F} {Imager} web-site},
month = {09},
url = {http://www.zf-laser.com},
urldate = {2013-09-18},
}
\end{filecontents}
\begin{document}
\citet{Imager2013}
\printbibliography
\end{document}

apalikestyle doesn't know what to do with theurlfield, nor withtimestamp. You could add it as anote:note={Retrieved on 18/09/2013 from \url{http://www.zf-laser.com}}Load theurlpackage or leave out\url. โ egreg Sep 18 '13 at 15:22biblatex. Are you particularly attached to BibTeX/natbib? (That is, would you accept a BibLaTeX solution?) โ Sean Allred Sep 19 '13 at 04:21