Questions: How do I register the target phone if the app is already installed and hidden?
Answer: You should show the app (show icon) by make a call with phone number 'access code'.
An 'access code' normally it is secret number you defined it in user control panel ( https://my.thetruthspy.com), click menu Device Settings \ Sync Settings and you can see access code. Also you can change it for more secure.
Default value it is #2013*
This is a link you can see for more detail: http://thetruthspy.com/faq/