Using actionwebservice and Rails 3

We want to use actionwebservice for some SOAP integrati开发者_运维百科on. We were able to install it however using it causes an error. Here's my code:

class HelloMessageApi < ActionWebService::Base
    api_method :hello_message,
               :expects => [{:firstname=>:string},
                            {:lastname=>:string}],
               :returns => [:string]
end

The error returned is

uninitialized constant ActionWebService

When I do a gem list I can see the gem installed, what am I missing? Please help.