you don't, and it would be a bad idea. Just set your password, then use it only for unlocking. Power button > secure device is enough the next times. It shows the lock pad but that's just if you want to unlock the device.
when you tap the button "secure device", it shows "device locked". When you type your code, you're back to the home screen and you can use the N900 again.

and I'd suggest a better password than 12345, too.