You'll probably need to go here for some JDBC drivers and some FAQs:
Unlike DB2 UDB for the other platforms, which have the JDBC driver packaged with the administrator client, you may need to get it from the Toolkit or get a third party driver such as HiT - depending on your performance needs. You may also need to patch your AS/400 to support external JDBC access. You don't want to use the native driver unless you are going to attempt to run JBoss on the AS/400 - I haven't heard of anyone doing this. :)
I'm not sure about having to rename the driver from a .zip to a .jar as Oracle and and DB2 UDB drivers have worked fine for me in the past.
you must use jt400.jar from JT/Open.
You can download from IBM, is free and open source.