To enable/disable or change the passcode you simply have to go through the setup again.
For version 3.7.0 or newer of the app first go to the 'More' tab on iPhone (this option is located in the left menu on the iPad).
Then choose 'Edit user settings'
For older versions of the app.
To do this first go to Account Settings (Located in the more tab on iPhone or in the vertical menu on the left on iPad)
Then choose 'Edit User Settings'
You now go through the app setup again allowing you to make changes.