For Windows
Go to C:\Program Files\Adobe\Adobe Flash Builder Beta 2\sdks\4.0.0
And duplicate the 4.0.0 folder and rename the new copy to AIR 2.0
Download the AIR 2 beta.
Drop it in:
C:\Program Files\Adobe\Adobe Flash Builder Beta 2\sdks\AIR 2.0
Within FB4 go to Window>Preferences>Flash Builder>Installed Flex SDKs
Click Add and navigate to the new folder you added.
It will ask you to give the new SDK a name (name it something like AIR 2)
Now create a new Flex project and choose AIR 2 (or whatever name you gave it) from the SDK drop down list.