My German is good for a tourist but poor for technical! I have used Google Translate to read your query.
I understand you have three questions.
1. All the Downloads are to be downloadable by Registered users only.
2. You want a User Group to be able to Create and Edit the Downloads from the Front End.
3. When downloading users have to acknowledge a licence.
For the first case there is an auxiliary question. Do you want non logged in users to be able to see the existence, description part, and similar? The ability to Download is controlled by the Permissions, and the ability to view in the front end is controlled by the view Access. The default installation of jD gives Public the permission to Download and Public view Access.
To turn Off Public Download go to the Component Permissions. This is simplest if you go to the jD Control Panel and use the Options button on top right of the tool bar. Select Public - you should see a pic like qaz01 below. On the Download pull down select 'inherited' - never select Deny as it has many side effects that are not expected - then Save. The Save is essential as it propagates the permissions.
Next Go to Registered and on the Downloads set as Allowed. At this stage Public can see but not Download, Registered and User Groups that have Registered as one of their fore fathers can both see and download. If you do not wish the Public to see then go to the jD Categories and set their Access to Registered. Also you could set the Menu item that shows the Downloads to a view Access of Registered.
There are several articles which may help in the section http://www.jdownloads.net/documentations/category/access-controls-and-user-group-permissions
For creating/editing Downloads from the front end I usually create an User Group (UG) called uploaderUG with Registered as its Parent. For this UG then set the Permissions for Edit and Create to Allowed, and Save. Next go to the jD User Groups Settings from the jD Control Panel. For UG uploaderUG make sure the Ranking value is set high at say 131. This means that all the UGs with Registered as their fore-father will use these settings. You should now look at Group Settings and Group Creation/Editing Settings to decide what you wish to allow. If you have specific questions please ask. The following may be helpfulhttp://www.jdownloads.net/documentations/item/introduction-to-user-group-settingshttp://www.jdownloads.net/documentations/item/which-user-group-is-used?category_id=29
In Group Settings and Group Creation/Editing Settings you will see a tick box called Licence, actually two tick boxes, the second one ensures that creating or editing means that a Licence must be selected by the uploader. Following is another tick box called Confirm License which means the User must acknowledge the licence before they can download.
Hope I have understood correctly.