Read the Knowledgebase on the Helpdesk
Hi FitzNS - I had a similar problem and decided to raise a ticket with the helpdesk.
Before going ahead with the ticket I decided to read the Knowledgebase section and found the answer to my problem:
If during the installation you selected to install only the core program, then you will need to re-install Trainz and install the full program. You will then be able to enter your serial number.
Cheers - Trevor