Two possible ways of handling this. First, when you purchased the progam, there should have been an e-mail sent to you, with an attachment.
Just double-click on the attachment, and that should solve the problem.
Just in case that doesn't work, take the e-mail itself, which has your key # enclosed by asterisks (**), copy and paste the number, highlight JUST the number (NOT the asterisks), into the 'Enter Registration Key' area in the 'About' screen. (click the question mark on the blue disc to locate 'About', click it, and you'll see where it has to go).
Good Luck and Happy Copying!