unable to use mock

[cranium2003]

hi,
  I am new to mock tool i followed whats given on wiki
of fedoraproject.org. According to that i set mock.
installed it. then i did su - build
then in build user i gave mock -r
fedora-5-i386-core.cfg foo-0.1.1.src.rpm
and i got output as
=============================
init
clean
prep
This may take a while
Error peforming yum command: /usr/sbin/mock-helper yum
--installroot
/var/lib/mock/fedora-development-i386-core/root
groupinstall build-minimal build-base build
ending
done
=============================
whats wrong happened??



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