Delete all local content of KA II you have on your rig right now - do it through Steam (Library/KAII - right-click and choose Delete local content)
Create a new steam account (or if you already have more use an account where KA II is not activated yet).
Games/Activate a product on Steam
Enter the code, download the Beta build. It is very important to delete all already existing KA II content from your computer before you do that!
And please use the Beta Forum for any beta related issues.