This could be because you have more than one version of Word on your computer, and are opening the document in a different version of Word to the one which you registered with.
License keys are valid per User, Computer and Version of Word.
If you only have one version of Word but you are still having this problem, you can Re-Register by going to VF Creator Licences on the VF Creator Software Registration page, then click Clear on the Key for your computer and Generate a new one, then entering that Key into the Registration box in Word.