You must enter your flight number on the INIT page before attempting CPDLC or PDC, otherwise you will not receive any replies.
To obtain a HOPPIE code you need to visit their website and register a free account. After completing a registration they will provide you with a code that you can paste to the fenix application. Fenix uses HOPPIE for CPDLC and PDC operations when flying online.
Registration link: https://www.hoppie.nl/acars/system/register.html
Please note: Your HOPPIE code must be entered into the FenixSim A320 app before opening the simulator.