Select Options, then Security.
Open General Settings
Next to "Password", highlight the word Disable and click the trackwheel.
Select Change Option, then Enable.
Press escape, enter save, it will prompt for a new password
Note: The password must be 4 to 14 characters in length. You must not enter a password composed of identical characters or a natural sequence (e.g., 1, 2, 3, 4), as it will be rejected.
Enter your password again to verify it.
Will set and go back to general settings.
You can also set Security Timeout, highlight the displayed time and click the trackwheel.
Select Change Option, and choose a reasonable time for the handheld to time out and lock. Its recommended 15 minutes or less.