Tools update today guys.
Just login to get it. It should download an update.
It should then auto apply itself then bring up the launcher. If you want to check if it's done anything check your Blizzard Updater Log.html in World of Warcraft Beta->Logs
The latest tools update I downloaded today was up the top under "World of Warcraft Update Tools (2.1.2.1542)".
Here is the log if you're interested.
Also if you are presented with a list of accounts to log into , choose the account for Cataclysm beta.
To figure out which open up a web browser to battle.net and login. You should see a list of games under "Manage Games". Look at the Box for Cat , the one with Deathwing on the box in some myspace pose (*hold camera above self* *strike a pose* *click*)
Immediately underneath that box will be either Wow1 or Wow2. To the left you may notice a box for Live with your live account name and maybe a box for the PTR. The account name to use for Beta is in the same place but under the box for Cata.