[Q] root sony ericsson arc s topic

im using generic flash tool to root my LT18a sony arc s
(version 4.0.4 ,kernel version 2.6.32.9-prf ,build number 4.1.B.0.587)
i receive error message :
ERROR - Error in processHeader : 2 : The system cannot find the file specified.
ERROR - Error flashing. Aborted

how can i root it?