Please ensure you are typing it exactly as it appears on the manual. It is easy to mistake a 0 and a Q for example and this will cause the serial number to be invalid.
If you have a digital version, please use copy and paste to ensure there are no typographical errors.
You may need to exit Trainz and restart it for a change to take effect.
Also, if you are trying to register an older version of Trainz DLC, it is possible that your serial number precedes the version you have updated to. i.e. your "old" serial number is not recognized by your "newer" version of Trainz.
In this case, please contact helpdesk and quote your date of purchase and your current Trainz build number (found on the Launcher).