LOL I'm an idiot.  Square brackets!

       public void Main()
        {
    MessageBox.Show(Dts.Variables["User::src_inbox_folder"].Value.ToString());
    Dts.Variables["User::is_new_file"].Value = System.IO.File.Exists(Dts.Variables["User::src_inbox_folder"].Value.ToString());

            Dts.TaskResult = (int)ScriptResults.Success;
        }