There is a strange bug between these two plugins. Just taking a moment to explain the error.
When both of these plugins are active, they work just fine (on the front end). If you attempt to edit a Ninja Form, however, you’ll see a blank page:
Simply deactivate Download Manager, and you will see all your fields pop back up on the desired NF form.
My guess is that there is some kind of jQuery conflict between these two. Deactivating DM takes care of it. Just don’t forget to reactivate it when done.