Your Key ID is used by Mechanical Simulation to determine your license type and maintenance status.  You will be asked for the Key ID when logging into the Users section of the website or when requesting technical support.

Dongle Keys

Your KEY ID is printed on the dongle.

All Users

From the browser (carsim.exe, bikesim.exe, or trucksim.exe) use the Tools->License Settings menu item to bring up a window showing your Key ID and other information.