DBMS on an iSeries is commonly referred to (but I think mistakenly called) DB2, it is not the same as DB2 on AIX or Z/OS.
Before you can access the datasource from Oracle SOA Suite 11g, you will need to download and install the JDBC DB2 for AS/400 from JTOpen.
The current Oracle SQL Developer 2.1.1 do not have support for DB2 for AS/400. Best tool for the simple data analysis is DBVisualizer, the free version but lean on features. However, if you want to perform data migration, export, complex SQL, etc you should checkout SQL-Workbench.
