Hi,
Ur problem is that the rollback segment u created is offline and should be brought online for use.In Oracle whenever u create a rollback segment it is offline by default.So u have to bring it online in order to use it.
The following syntax will bring the rollback segment online.
SQL>ALTER ROLLBACK SEGMENT <segment_name> ONLINE;

U can place the name of the rollback segment in ur init.ora parameter file
ROLLBACK_SEGMENTS=(rbs1,rbs2,rbs3) and restart the instance.
If u do so then when ur database is started the named rollback segments will automaically be brought online,instead of bringing them online each one-by-one

If u have any doubts,please be free to ask me at
[email protected]

Regards,
Rohit Nirkhe,Oracle DBA,OCP