Hyperveloce
The CPDLC logon is really simple and takes very little time. The pilot enters the ICAO identifier on the ATC LOGON/STATUS page of the CDU and presses SEND. The result does not take continuous monitoring. This page is shown below.
Tha ADS-C contract is set up by the ground station. The flight crew have nothing to do with it.
What we do not know is if AF447 attempted to contact DAKAR on HF. And we will not know until the CVR is recovered. It is possible that any attempts to contact DAKAR may have been recorded on the ATALANTICO tapes, but we don't have those either.
For those interested in CPDLC and ADS-C there is very good coverage available at
ATC DL News Home