To activate it in the App » tap the gear-wheel icon IMG#58371 » scroll down » activate the slider next to 'TouchID' » enter a new PIN and confirm PIN. The App is now TouchID activated, so you'll no longer be asked to enter your password for any installed profile.
To deactivate TouchID » open the App » tap the gear-wheel icon IMG#58371 » deselect the slider next to 'TouchID'. You'll be asked for a final TouchID verification. After the verification, TouchID is turned off in the App.
Keep in mind that if TouchID still can't read your fingerprint after 3 attempts, you'll be asked to enter your PIN. If the PIN is correct, TouchID is again activated. If you no longer recall your PIN, you'll need to delete the App from your device and reinstall it. Unfortunately, all of your unsaved messages will be lost. That's why we suggest you choose a PIN that you'll easily remember.