Go to IS admin page and click on JDBC pools under resources. Configure the ISInternal functional alias and use the test button to make sure your IS internal table can be reached by IS and if it succeeds then restart the IS. If the test fails then
1.check if you can reach the DB from the IS installed box (go to cmd prmpt and do a ping or TNSping for that DB host).
Check the URL that you give for the alias (refer IS Admin guide).
Make sure you have the latest JDBC driver under IS/lib/jars (optional step).