There's no such thing as a "KLAX V1" or "KLAX V2". In FSX and P3D there's only one KLAX, and it's V2, which is called this way in order to not have user confusing it with the old KLAX from Cloud 9, which was for FS9 only and, since KLAX V2 works in FS9 too, we need them to have different names.
So, if you use FSX or P3D, your KLAX can only be V2, so there's no reason why you Serial Number won't be accepted, unless you are *really* trying to use a Serial Number for the old Cloud9 KLAX.
You don't say anything about the error message, so it's not easy to help you. Of course, such kind of support should always use email, not the forum, since in order to help you better, we might have to discuss your serial number so, please, use the support email, not the forum, to discuss anything related to orders.