Peter,
First you must register on the website
https://invenio.bundesarchiv.de/invenio/login.xhtml
After registration you have to go to 'Suche' :
https://invenio.bundesarchiv.de/invenio/main.xhtml
In the field 'Signatur' one has to mention the requested file and then 'Suchen'.
Good hunting !
Best regards
Luc