Hi, there.
When you launch VoiceAttack, you'll be presented with a screen that has a, 'Register' button at the bottom. If you've pressed, 'Go', you can still register by going to the Options screen (via the small wrench icon on the main screen), and then clicking on the, 'Registration' button on the General tab.
If you did not see either of those, you've probably installed the Steam demo and decided to purchase the standalone version from the VoiceAttack.com website instead. You'll want to uninstall the Steam demo from the Steam client and then install the standalone version from here
http://voiceattack.com/download. The version of VoiceAttack on Steam doesn't deal with registration keys at all (just relies on the Steam client), which is why there is no place to enter a registration key.
Hope that helps!