To create a Dropbox in D2L, go to the "Course Materials" section of the menu bar, then select "Dropbox" from the drop-down menu.
Then click on the blue "New Folder" button (pictured below).
From that screen, at the very least you need to name the Dropbox. If it's associated with a grade item, category, group, or rubric, add those specifications under the "Properties" tab and save with the blue button at the bottom. Also, follow the directions below when you are ready for your students to view the folder.
When you create a new drop box folder, you can immediately change the visibility setting on the restrictions tab. This is also where you can set a start, due, and end date. If you have the "hidden" box checked, you must uncheck it in order for the start and end dates to work. Visibility status overrides the dates. 
If you have the "hidden" box checked, you'll be reminded of it by the "eye" icon in your list of dropbox folders.
To quickly set visibility, you can click the dropdown menu next to the folder in your list and choose "make visible to users."
The "eye" icon will disappear, indicating that the folder can be seen by students.
You can also reverse the visibility quickly, using the dropdown menu and choosing "hide from users."
