You need to add the database you have created to the "Edit Configuration" program using the NEW button.

You need to use the "Edit Configuration" version that matches the Revit version.

The link below provides more information.

https://knowledge.autodesk.com/support/fabrication-products/learn-explore/caas/CloudHelp/cloudhelp/2...

or this link:

https://knowledge.autodesk.com/support/fabrication-products/learn-explore/caas/CloudHelp/cloudhelp/2...

It also needs a GUID:

Set a GUID for a Database | Fabrication Products 2019 | Autodesk Knowledge Network

Configure the CUSTOM database in Fabrication CADmep First. Once it is loaded completely. Open Revit Fabrication of a similar version. The Custom database will show automatically.

Only one condition: the version should be the same for Fabrication CADmep and Revit.

You need to have the fabrication database pathed in the same year of Fabrication CAD before you can load it into the same year of revit. it is important that the year of revit and fabrication CAD match or you wont be able to load it in revit.
once you have the database pathed in CAD, navigate back to the fabrication settings in Revit and the option should be there for you to add the database.