You need to go under the Tournaments Section and then under the Ready-Go section. Just look for the tournament with that ID # (it should have a red exclamation point next to it if you've haven't played it yet) and click the PLAY NOW button that appears to the right when you highlight #1231.
You can also launch the round from the client menu by going under TOURNAMENTS > READY-GO. Same deal there just a different interface. Look for the ID# or the red exclamation point.