You asked for a way to make the "free download work." But here is the cold truth:
OMSI 2 Add-On Urbino Stadtbusfamilie is a commercial DLC and is not legally available for free download
: High modularity allowing for individual configurations and full "repaint" capability to match your virtual bus company.
You can find the add-on at authorized retailers. Standard pricing is typically around , though discounts are common. OMSI 2 Add-On Urbino Stadtbusfamilie - Fanatical
Search reputable OMSI community sites and forums for the Urbino Stadtbusfamilie pack or the author’s page. Look for:
: Three-door and two-door solo buses, plus four-door and three-door articulated versions.