Redmine

hello,

i installed redmine in my network V2.0.3.stable i need to know the steps for installing redmine_backlogs plugin to it. i tried to install it but it gives me this error when i try to run this command bundle exec rake redmine:backlogs:install and the error that appear is ERROR: While executing gem ... (RuntimeError)
Unknown command nokogiri

although i installed this file nokogiri and this is the version of it nokogiri (1.5.9 x86-mingw32) it gives me this error rake aborted!
No such file to load -- nokogiri

redmine installed on windows 2008 R2

i updated the gems

i just need to know what i missed to complete installation for this plugin

thanks,