yes,I use fc6. I got the key from
rpm --import http://archives.fedoraproject.org/pub/archive/fedora/linux/core/6/i386/os/
and then I did: yum in stall fedora-ds ,but there is the same problem!!
1- in above link there are RPM-GPG-KEY-fedora
and some other keys which one should I use? only RPM-GPG-KEY-fedora?
2- in http://directory.fedoraproject.org/wiki/Download ,in addition of the key problem, there are some problems for centos5  . should I do those too?
 for example:
yum install svrcore mozldap perl-Mozilla-LDAP libicu