You need a JDBC driver to access the database. If you are running InterFormNG on Windows, then you can place the driver in the directory: C:\Program Files\Java\jrexxx\lib\ext (where xxx is the Java version) or in the directory: C:\Program Files\AdoptOpenJDK\jdkxxx\jre\lib\ext, if you use OpenJDK (where xxx is the version).

On Fedora Linux with OpenJDK that would be something like

cp /usr/share/java/mysql-connector-java.jar /user/lib/jvm/jre-1.7.0-openjdk.x86_64/lib/ext

You can download the JDBC-driver from MySQL's homepage.