You need the relevant drivers installed and configured on your server. DriverManager does what it says on the tin, but can only manage drivers it knows about.
In the first instance it will be making reference to the jdbc.drivers system property. If the correct class isn't referenced there, or you can't manipulate this property, I would suggest loading the class explicitly, e.g. with something like this:
Class.forName("com.ibm.as400.access.AS400JDBCDriver");