Open the game's data\database folder. There you'll find the 'default' folder, which is the official database. Make a copy of it. In the new folder, open the 'eras.xml' file. I recommend using an advanced text editor such as Notepad++ when you need to edit the files manually, so the file contents won't be messed up. In the .xml file, you see this line:
<string name="databaseID" value="default" />
You must put your database ID there. For example:
<string name="databaseID" value="myDatabase" />
After this, when you run the settings launcher, you should have your database selectable in the database dropdown menu. Select it, and go to the in-game database editor, and the content should be editable there!