first of all the server role that's usually used for dumb file servers is a "member server", not a "domaincontroller slave". The "domaincontroller slave" is designed to be able to function as a login server for Windows and LDAP authentication server for all kinds of services. Its usual place is as the only local DC in a branch or site office with the DC master located in the main office. You can use a DC slave for file sharing, of course, but it's usually overkill (and I'm a fan of having as little complexity as possible).
That being said: Enter the Univention Management Console on the DC master (not on the file server) as a user with administrative privileges (e.g. "administrator"). Navigate to "domain" → "shares" and click on "New".
In the following dialog you're asked for where to store the LDAP object containing the share's configuration. You can leave it at its default & click "next".
Now you're in the dialog allowing you to edit the share; the "general" tab should be shown. In this tab there's a setting called "host" which should list all servers on which you can create a share — and this particular drop down box should include your new file server. If so select it and fill in the rest of the required data. Shortly after saving this configuration the share should become available on the file server.