Code signing is required for applications to run on devices (as opposed to simulators).
You can implement code signing from the Device Application Designer or the BlackBerry JDE:
Device Application Designer – the BlackBerry generation wizard includes a “Disable signing" option. If this option is unselected the wizard presents a signing dialog after compiling. This dialog allows you to input a sign key to sign the options and the client COD (.cod) file, so they can run on a physical device.