To connect to a Database in custo diagnostic 5.x you need to have to download SQuirreL SQL Client, you can do so from here: https://sourceforge.net/projects/squirrel-sql
Now shutdown the Apache Tomcat diagserver from the Services menu in Windows.
Start the SQuirrel SQL Client and enter the settings to connect to the database.
The DbUrl can be found in this file: "C:\Program Files\custo diagnostic server\conf\diag.properties"
Now you can open the database and look into it.
For a more detailed instruction look into this VIDEO