When DN75 is available for existing clients a large 'Update' button will appear on your dashboard. After you click that your site will update.
We expect this to be available within the next two weeks. Existing clients are slightly delayed while testing continues in some core areas such as converting existing designs to SVG. New clients or clients with little designs or decorated products are mostly not affected so can use Dn75.
The Define New Production action was discontinued from Business Hub due to incompatibility. In Business Hub, you have access to use Free-Form products as a way to add in infrequently ordered products that are not already in your system. Aside from that, you can still create custom products but it must be done in the Admin section (Admin > Products > Add Product > Add Custom Product) where you can have full access to the configuration options for a new product.