Hi Railsignal,
First, welcome to Trainz and the Trainz Forums.
You did of course keep the email that came with the authorization to download the product... so looking for the email would be a starting point.
Hopefully, you registered the serial number with Auran...so...if you look upward on this screen, you will see the words...SERIAL NUMBER...right between CODE OF CONDUCT AND WEBSITE.
Again, if you took the time to register, it will be there. Otherwise, you only option will be to contact the Help Desk...and it is the weekend in Australia, so you may not get an answer immediately.
HTH and regards,