You must be looking in the wrong place. When you go to Components -> DT Register -> Registration Management, you will see a list of items that have already been setup in DT Register. The dropdown on this screen is a FILTER of existing DT Register items. Your events from JEvents will NOT show up here... only DT Register items. This is a manner of filtering your events to find what you are looking for once you have many registration items setup.
To setup a new registration item, click the NEW button in the top right corner. The dropdown on this next screen WILL show your JEvents items for you to pick from to add registration details to the events.
Viewing our video tutorials will help to avoid things like this...