The second arena is built and the sand is ready, but it needs a client update first, so that signing in through Steam and buying with the Steam wallet work here exactly as they do on the main world. Opening it before that would hand you a world where half of it does not work.
The update goes out tomorrow and this world opens with it. The address will be waiting in your in-game mailbox.
Until then the arena is where you left it: lanistaworld.com. Nothing there changed.