please always(!) copy the mailing list on your replies.
I am using Lammps (Apr 7 ,2010) version.
there you are, your version is over a year and a half old.
the online documentation of LAMMPS, however, always
represents the current version. very obviously the command
you want to use is either not implemented in your version
or has a different syntax, so you either have to upgrade
your LAMMPS version to the current version, or consult
the documentation that matches this version (usually
shipped with the source package).
axel.