This option is available only in the paid version of the app.
Go to the main screen (where all your notebooks are) and tap “Edit” in the upper right corner of the screen, select the notebook you want to assign a passcode to. You'll see a lock icon in top left. Tap it and enter your passcode in the popover.
Note: Due to security and privacy reasons we don't store your passwords and there is no way to recover them. That's why it is recommended that you send your passcode by email – the option is offered instantly after you've set a passcode.