The license is stored in the Windows Registry. Anyone familiar with the "regedit" tool should be able to locate it.
The license key is not exposed by the software, to keep it more secure in an environment where there could be any number of people accessing the PC running the tournament.
When you place an order for a TD license, the license is emailed to you, with instructions to hang on to it in case you need to re-enter it at some point. In our world of hard drive crashes and virus, spyware, etc, this is fairly common. So you should really hang on to it. If you cannot locate it, email support@thetournamentdirector.net and I will see if I can look it up.