The IBM DB2 thin driver is not packaged with TIBCO Runtime Agent. You must add the driver files under the install-path/tibco/tpcl/n.n/jdbc folder.
The default page size for DB2 is 4k and this is not enough for creating a domain. You must create a bufferpool with 32k and then create a tablespace that uses this bufferpool. The table space must be made available to the given DB2 user.
After making the changes using Domain Utility you must complete the following: