It's happened before. The first validation code I got for my copy of QXP 7.x was wrong so I called tech support at 5 mins to 5pm and they sent me a new one – which worked.
In any case...
Validation code: This is the long string of letters/numbers you enter when you validate the install.
Activation code: A series of numbers that you get from Quark to activate your copy of XPress AFTER it's been installed.
Use the validation codes when the installer asks for them. You can keep the activation codes handy if you want to, but it only really applies after XPress has been installed. When you open XPress for the first time after the install it's usually easier to just let it activate over the internet then it is to try and enter 47 numbers.
As far as I know too the validation codes are also country specific. So if you happen to be calling Quark in Denver and you live in Nottingham England (just as an extreme example) you are probably getting a US validation code which wouldn't work on your copy.
Hope that helps.