Did you read the text immediately under the link you're trying to click? It specifically says what's going on.Originally Posted by MINTY
The link can only be used once. The purpose of the link was to create your ZBrush 3.5 account. Now that you have an account, the link is meaningless.
After you upgraded, you got a second email from cleverbridge. This one has your reference number in the subject line. It has your serial number. And it has a download link that CAN still be used.