Hi,
Please first create an ODBC Bridge using the correct DB2 Driver, for that you will have to download and install the db2 client, then you can catalog the node at your end and register it for odbc.
Then using the get external data source option you can connect to db2.
Please let us know your db2 version, fix packs applied and operating system in subsequent posts that does help us a lot to understand your problem better.
Cheers
Nitin.