[@Dave]
Dave, I am Elena from HT Vector support.
It could be two reasons
1. You mistyped the key.
2. You tried registering the regular version from ht-vector.com.
The key is valid only for the version you can download from Shareware on Sale. Please make sure that you download the correct version and enter the key exactly as it was given.
Please write to [email protected] if the problem persists.