I've installed CadStd Lite on my computer and it didn't request any password entry before or after the main installation. What you refer to might be the serial number which is needed for the Pro version of the software. You will have to contact the developers, because they're the ones with access to the serial generator. When contacting them, make sure to also provide the buying information you previously used when the software was purchased for the first time.
Use the following page to send and inquiry to the developers: http://www.cadstd.com/customer.html