The ieeetr
bibliography style is one of the original BibTeX styles. It dates back to (at least) the early 1990s. As such, maybe unsurprisingly, it's not programmed to do anything with a url
field.
I suggest you use the IEEEtranN
bibliography style instead. It is much newer and does know how to process the url
field. It also doesn't ignore the address
field. If you follow this approach, you should load the natbib
package with the option numbers
. In addition, you may want to drop the string "Online" from the note
field, since the string "[Online]. Available:" is prefixed to the url
field by the IEEEtranN style.
A separate remark: I'm afraid I don't understand a single word of Finnish, but I suspect that one or more of the words Vuosineljännes
, Liitekuvio
, and Sähkön
should not be converted to lowercase by BibTeX. If this hunch is correct, you should encase the words in curly braces, to exempt them from the "sentence style" conversion that's applied by the IEEEtranN bibliography style to the contents of title
fields.

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents}{task.bib}
@misc{ stat_suomi_90_14,
author = "Tilastokeskus",
title = "4. Vuosineljännes 2014, Liitekuvio 12. Sähkön tuonti ja vienti 1990–2014*",
address = "Helsinki",
url = "http://www.stat.fi/til/ehk/2014/04/ehk_2014_04_2015-03-23_kuv_012_fi.html",
note = "visited 6-May-2015"
}
\end{filecontents}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{url}
\usepackage[numbers]{natbib}
\bibliographystyle{IEEEtranN}
\begin{document}
\nocite{*}
\bibliography{task}
\end{document}
ieeetr
bibliography style, or are you free to use one of the more modern styles distributed by the IEEE?@online
?); (2) use a.bst
that uses those fields for the@misc
entry type; (3) copy, rename, and then modify theieeetr.bst
so the@misc
entry type prints those fields; (4) dump all the information you want to get printed into thenote
field and format it all by hand; or (5) switch tobiblatex
(which opens a whole new vista of possibilities, but doesn't necessarily make it easy to get anieeetr
clone without doing the work yourself).