Splunk Free does not have any user authentication or roles. So I don't think the add-on will work with Splunk Free.
You might be able to use just the Splunk Mobile App without the add-on. Have you tried that, just pointing it to the URL of your Splunk Free instance? See Sign in to the Splunk Mobile App for details. I don't know if it will work, but it should only take you about two minutes to try it....
