It's not open to the public currently, you would need to be invited. I read that after Craglorn is released they are going to be looking to open it up to the general public.
Worstluck - Breton Nightblade"Some of us refused to bow. We knew the old ways would lead us back to having a kingdom of our own."
―Madanach Elfluck - Dunmer Dragonknight "When I will walk the earth again, the Faithful among you shall receive your reward: to be set above all other mortals forever. As for the rest: the weak shall be winnowed: the timid shall be cast down: the mighty shall tremble at my feet and pray for pardon."
―Mehrunes Dagon Deadluck -Imperial Templar "Men are but flesh and blood. They know their doom, but not the hour"
―Uriel Septim
It is an invitation only public test server. Basically they want people they know have given them good feedback to do the testing.
Best way to get invited is submit detailed bug reports when you find a bug and thoughtful /feedback on problems you encounter. Doing that sort of activity will generally bring you to the devs attention in a positive way.
You're not missing anything anymore. It was awesome back when the game wasn't out and we had access to it 24/7 when nobody else did, but now its probably more worth the average player's time to just stick to the live servers.
Craglorn should be hitting those in a week or so anyway.