The activation key is matched to the serial number of the PIX (taken from a "sho ver" output), you would have provided this serial number when you applied for the activation key. Make sure the one you provided is correct, and don't use the serial number on the back of the chassis cause if you've ever upgraded RAM on the PIX then this number will change, so make sure you provide the one from a "sho ver" output.
Other than that it should be as simple as running the "activation-key" command and putting it in. If the PIX is saying it's invalid, you're either typing it in wrong (which I'll assume you'v echecked :-) ),, or the serial number doesn't match.