You also can check the No.2 disclaimer, you don't need retail wow copy to play the Beta, the beta itself is a standalone build.
The first thing is to check,if you got those kind of cata invite from e-mail,login to ur battlet.net account 1st and check normally if u confirm got the invite, there will be wow2 account displaying cataclysm list in ur games.If not,confirm that e-mail is fake.