After importing a race as described here, you can very easily import your participant data.
First, you'll need to enable Participant Syncing by letting RaceDay Scoring know what (if any) additional fields you would like to pull into RaceDay Scoring from RunSignUp.
To get started, click the Participant Sync sidebar item:
On this page, you can select what information should be imported.
If you would like to, you will be able to import additional data, like custom question responses or Emergency Contact Information. This process is explained here.
After adding in Field Names for any data that you would like to pull in to RaceDay Scoring, click "Save Sync Settings", and your participant data will automatically begin syncing in the background.
At this point any changes to participant data on RunSignUp will automatically get pulled into RaceDay Scoring, and any changes to participant data in RaceDay Scoring will automatically push to RunSignUp.
If you do not see your data populating, be sure that your system is online (using the top bar status indicator), then try using the force sync button in the race name toolbar: