Only so much that should be said on a public forum.. we don't want kids getting lost down there.
What I will say is that it's the same key as for Backdoor and the location of Jack's can be found with a bit of research on the internet. Jack's puts you into the southern region of the mine which is a different survey map to the central area where the Backdoor is. You can buy both surveys from the Quarryman's pub. It's a long trek underground from Jacks to the famous sights of Box Mine, so take care, make sure you have a compass and plenty of spare light and only go in as far as you feel comfortable with. Ask at The Quarrymans and they can put you in touch with local guides if you are lacking confidence for a self-guided long trip underground.