ulog is installed by default on the drone, you don’t need to to anything to use it.
If you’re not seeing the logs from your mission in the missions-data/FDR directory, you check the following:
Is the AirSDK mission log settings properly enabled ? You can check and set this with FreeFlight.
Is your mission properly loaded (and/or activated) ? If you installed your mission without the --default flag, the mission will not be automatically loaded (see the mission lifecycle documentation for more informations).
The behavior is the same for “Full missions” and “Service only”, so yes you will need to use either the --default flag, or a custom Olympe script to load it.