When creating new Schedule Item, I am unable to schedule across multiple rooms as Room field will not display as checkboxes

Hi -- I am flummoxed, and hope someone might know what the problem is. I'm using COD, for Drupal 6, for a conference web site, and while the documentation says I should be able to schedule an event across multiple rooms AND while I have the Room content type set so that it should display as checkboxes/radio buttons, when I actually go to create a Session or Schedule Item, the Room field will only display as a Select List. Has anyone else run into this? Or have an idea of how to fix it?

Thanks so much for any ideas you can share!