When i tried to instail this plugin
http://fornwall.net/eclipsecoder/ to compete in Arena at TopCoder
I found this problem.
http://i.stack.imgur.com/Vvc
I think that you have invalid update site URLs in the available update site list (Window > Install/Update > Available Software site). Remove or disable the update site causing problem and retry adding the proper update site URL you want to access: http://fornwall.net/eclipsecoder/
You get this error when p2 can't find the update site to install. Looks like the url that youa re using is correct, so the problem is one of two things:
As a side note, the update site is an old style update site (ie- pre-3.4 and does have proper metadata). You should encourage the maintainers of this plugin to update to a modern update site.
I don't have any problems installing the plugin. May be the location you have specified when adding the repository was wrong. The location specified in the site of EclipseCoder and that I use is: http://fornwall.net/eclipsecoder/