The wireless password is a property of your wireless router, not your wireless printer.
Therefore, you'll see it in your router's web-based configuration utility, in the wireless section.
Dig out your router's setup guide and it should tell you how to access it's configuration utility on a PC or Mac using a web-browser. If you can't find it's setup guide, go to your router manufacturer's website and click "Support". Specify your router model and you should be able download it's setup guide or user guide.