Ruby on Rails - RESTful API Authentication

Im building a simple REST API with ruby on rails that allows users to easily create an account. Recently I have tried to secure the API with an API token. I have been testing my API with the Advanced Rest Client that is found on the Google chrome web store. As far as I know it has worked as when I try to access the calls I get a message from my REST client test application that looks like this:

The problem comes when I try to authenticate successfully with the Advanced Rest Client. Here is the controller code that I use to create the token and check to see if the user authenticated:

class Api::UserController < ApplicationController
  skip_before_filter  :verify_authenticity_token

  TOKEN = "secret"
  before_action :authenticate

  def index
    @users = User.all

    respond_to do |format|
      format.json { render json: @users }
    end
  end

  def create
    @user = User.new()
    @user.first_name = params[:first_name]
    @user.last_name = params[:last_name]
    @user.email = params[:email]
    @user.password = params[:password]

    respond_to do |format|
      if @user.save
        @response = {:status => "201", :message => "User successfully created."}
        format.json { render json: @response, status: :created }
      else
        format.json { render json: @user.errors, status: :unprocessable_entity }
      end
    end
  end


  # private methods in the UserController class
  private

  def authenticate
    authenticate_or_request_with_http_token do |token, options|
      token == TOKEN
    end
  end
end

It's fairly simple and seems to be doing its job as I haven't been able to authenticate with the Advanced Rest Client. I have tried to set an API token in the HTTP headers but can't seem to get the syntax correct. Here are a few examples of what I have tried:

and,

and finally,

What am I doing wrong? Is the API token not stored in a HTTP header? Please help!