def help
    Helper.instance
  end

  class Helper
    include Singleton
    include ActionView::Helpers::UrlHelper
  end

  def add
    @product = Product.find(params[:id])
    @item = @cart.add(params[:id])
    flash[:cart_notice] = "Added <em> #{@item.product.name}</em> to your #{help.link_to "Shopping Cart", :controller => "cart"}"
    redirect_to :controller => "products"
  end

#############

You have a nil object when you didn't expect it!
The error occurred while evaluating nil.url_for