To validate a registration key in VoiceAttack, you'll want to first launch VoiceAttack. You'll then be presented with a screen that has a, 'Register' button at the bottom. Click the, 'Register' button and you'll then be prompted to put in the registration key and the email address you had used to purchase that key.
If there is a link in your email to register, it's probably not an email from VoiceAttack.com.
Hope that helps!