if it's an AS400, then you should use JTOpen libraries to connect.
JT400 has the JARS you need to use. Note, the class name mentioned above is not the same as what is expected on the AS400....
I remember it being something like this....
com.ibm.as400.access.AS400JDBCDriver...
you'll want to double check
if you start squirrel-sql
on the commandline you can add this
set CLASSPATH=%CLASSPATH%;c:/jt400.jar