Check In is not available in Cloud for the individual crewmembers home PC.
To be able to check in from a PC the IP address of the PC must be registred.
see Checkin IP Address
The conditions for Check In is defined in the Setup parameter "CheckInStart" and "CheckInUsePositionData".
When a flight is open for Check In the crew will have access to the check in button
Click the "Check In"
and the system will confirm the check In.
In the crew gant you will see a green triangle indicating that crew has checked in.
Missing CI time can be monitored by a rule and a Task filter
The Crew Controller can manually Check In the crew