Hi kenwtn
The serial number will have been listed in your order history when you purchased the digital download. It will also have been in the email that was sent to you, and that you will have used to 'activate' TS2009 when you first started it. It will start with 'ATS9'.
To use the download station, you will need to register the serial number in your profile. You can do this by clicking on 'serial number' on the top toolbar of the forums, then entering your serial number into the boxes provided.