install plugin in rails 3

To install error_messages_for
i run this commond but it does'nt work

rails plugin install git://github.com/rails/dynamic_form.git

please help me out

i am using windows platform

what error you are having?

when i type this command rails plugin install git://github.com/rails/dynamic_form.git

it show

usage:
Rails new APP_PATH [option]
options:
-r , [--ruby=path] # path to ruby binary of your choice

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and something like this

You should use this command inside your APP, which is what I did but all I got is an empty directory.