There interview scheduler just works for the oncampus one.
From what is stated on the site. They understand that it is not possible for everyone to visit Darden for interview. For such applicants their advice is to submit the application without interview and if the ad com finds it necessary to have a talk with you, they will contact you.
I was thinking of sending them a mail just to show my eagerness towards the program.
Should I or should I not?
I never see the harm in being a little persistent as long as your not being overbearing. An email re-expressing your interest is not a "bad" idea, just keep it short and sweet.