A legit email from Blizzard wouldn't require you to do anything, the beta license would just be there.
A 3rd party contest of some sort would require entering a key (your original post implies you entered a key) in those cases it could be legit.
Log in to your battle.net account and look at the WoW licenses, if you have a legit beta invite added you should have 2 or 3 WoW accounts listed, the normal live one, and 2 PTR ones, the last of which would be the beta account (for me it's listed as WoW3).
If its there just click on it and there will be a link to download the game client and another link saying "beta character copy" which you use to copy one of your existing characters to the beta servers.
just my original account and 1 PTR account which says wow1.
not played wow for over a year, so when I got this key, I thought what the hell, lets have a go.... I'm not too fused.
thanks for the help all