The Oengus Schedule Import panel is part of the nodecg-speedcontrol bundle and has not had any modifications from its public version. Due to Oengus' ability to add custom data to runs which the STUK Scheduling Documents make use of, it's highly recommended to use Oengus for scheduling your marathon.
The import panel is pretty straightforward, add the marathon shortname and schedule slug of your Oengus marathon into the boxes and then click the Import Schedule Data button. You can pre-populate the shortname and slug by entering them into the relevant fields of the nodecg-speedcontrol.json config file. An alert dialog will appear to confirm that you want to continue, and after a short loading period the runs will load into the speedcontrol run editor.
It is worth noting that this import will overwrite all existing data in the run editor - if you import your Oengus schedule, add extra details to the runs, then import the schedule again, the extra details will be lost.
Using the Oengus custom data field, you can add each run's Layout Code, Commentator and Host details, so these can be retained when importing. Unfortunately, there isn't currently a way to include any runner custom data (stream key, camera usage) in this field. Therefore it's advisable to complete your Oengus workflow as follows:
Update Oengus with any layout/commentator/host custom data as it becomes available. If anything in your schedule changes, update it on Oengus and then re-import periodically.
As you get closer to your marathon start, declare a cut off where Oengus will be imported for the last time. Any schedule changes that take place after this cut-off should be updated manually on both Oengus and NodeCG.
Only after the cut-off should you add any runner custom data, or edit any other runner details. This way, this information won't be overidden as you're no longer importing from Oengus.