Another way of thinking about this... to have a functional CPDLC, you have to have something for it to communicate with. PMDG and POSCON (link provided as I know some people have broken Google Searches... LOL! https://poscon.net/) will be providing this, so you'll have to ask them when it will be functional. Otherwise, it's like having just one telephone... you can talk into it all you want, but unless there is someone else with a telephone your conversation will be one sided!
No, CPDLC will not be functional yet. It will be in some form with the PMDG add in. But apart from logging in to KUSA and getting a departure and enroute clearance I think PMDG will model nothing else (for now).