you can't they are closed
in a closed beta you will be able to visit the beta website and apply for a key, the website will ask you to download a program to asses your computer specifications and then if your computer is good enough they will choose you.
This depends on the type of beta: open or closed. An open beta is open to anyone. Most likely you can simply visit the website of the game or the game company and download the game to try it. A closed beta, however, is by invite only, or sometimes limited in total number of people. For this you may have to already know the game publisher (signed up for an account on their forum, subscribed to a newsletter, purchased games from them before, etc) to receive an invite, there may be contests to get an invite, or if there is a limited number of downloads, simply download the game quickly before the beta is "full" (no more people allowed).
If you only just asked it ( 28.7.12) or only about 6 or 7 months before, it's because the beta has closed.
The closed beta is already finished so everyone can play just go to http://sonic2online.com and download the game.
The closed beta has already ended. The open beta has been released, and is becoming increasingly available across the world.
The only way you can do that is to look for new MMO's that are in BETA, after BETA is closed, you can play it. Usually, some BETAs require you to reach a certain level or something to keep you character in the main game.
The closed beta has ended, but you can participate in the open beta by downloading it from the App Store or Android Market for free, provided it is available in your country.
Well there is Roblox,Lego Universe,Brick Force(although it's still in closed beta),Block city,atmosphir and Minecraft.Those are the only ones that I could think of off the top of my head.Happy gaming!
No beta
Go to this website to download Pivot Beta 3: http://www.softlist.net/search/pivot-3-beta/
There was a beta program for LU but now that it has released, the Beta is closed.
Cataclysm will be closed beta, meaning you can not play it without a key that Blizzard gives you. The beta will act like the PTR, but will be seperate. For more information look at the site in the related links