IBM DB2 iSeries and ZSeries Z/OS are not supported at this time.
To connect to DB2 LUW, SQL Developer first needs the correct JDBC driver.
Only one specific JDBC driver is supported.
db2jcc.jar with db2jcc_license_cu.jar
All other IBM drivers are not supported and may cause problems even if the above two drivers are specified. So best to only add the above two.
These drivers are usually found in your own DB2 database install or DB2 client install.
They are also available when you agree to the license (please check this first) and download IBMs DB2 Express-C database or Data Studio Standalone administration tools
Once you have a DB2 client or database installed you can search for the specific JDBC jar files.
Heres what my preference page looks like. (note, JTDS is for SQL Server and Sybase)
Once the correct jar file has been added you now have an extra DB2 tab in the connections dialog
Once you add your connection details you can browse your DB2 database objects.
You can learn more about migrating to Oracle from IBM DB2 LUW here