Version 2.8.3

I have purchased and installed 2.8.3, but when I entered the Serial Number:
[redacted]
a dialog box appears telling me this serial number is invalid for 2.8.3.
Can you help me through this impasse?

That serial number is correct; it should work if you enter the Serial Name as shown in your order confirmation e-mail. The easiest way to enter the name and the number is to open the Serial Link in that e-mail.