In Blackboard 9, the Dropbox tool has been replaced with the Assignment tool. If you want to modify your courses now to use the Assignment tool, I have created two videos that will show you how to create an assignment, and another video that you can link to for your students to them how to submit an assignment in Blackboard 8.
Creating an Assignment
Submitting an Assignment
Of course, this process has changed quite a bit in Bb 9. Here is a PDF created by Blackboard to show you how to Get Started with Assignments in Bb 9.