I created some new fields after I built the event. I am unable to make the fields show in the event. Only solution I found was to create a new event with the new fields. I was able to change existing fields with their changes taking effect immediately .
You can certainly add new fields to an existing event. Make sure you re-save the event and verify the settings for the new fields on the "Fields" tab of the event setup.
It seemed strange to me, I am on DT Register 2.7.9c and Joomla 1.5.23. I created the new fields and went to fields in the event. It did not show the new fields. I looked to see if then could be on an additional page but found nothing. Creating a new event was my only solution.