Hi Pha
Did you add 0COSTCENTER as an InfoProvider ? RSA1--->InfoProvider . Search for 0COSTCENTER.
If you did not get it there, Right Click on any InfoArea and select "Insert Characteristics as InfoProvider"
You should be able to see COSTCENTER in BEx .
Additionally from transaction code SE16 check if you have data in table "/BI0/PCOSTCENTER"
Cheers
Anindya