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 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/