just HOW did you install it ?
cent5 uses qt3 and kde3
and has qt3-designer in the repos
also 5.5 is no longer supported 5.6 is current .So did you upgrade ?
and have you looked over the wiki ?
for a TESTING ONLY kde4 and qt4 install on Cent5.6 see
kde-redhat.sf.net - This repository provides more recent (bleeding-edge) versions of KDE for use on Red Hat based systems. This repository has a decent reputation and a sizable support community.
Note: - This repo will update MANY packages to newer versions than Base CentOS ships, including all of KDE, QT, samba, etc. A CentOS team member commented at revision 25: "This seems to work OK on all machines I have tried but your machine will be far from CentOS with many changes to the [Base] CentOS Operating System." Information concerning setup is available here: http://kde-redhat.sourceforge.net/
most is a bit out of date
fedora 8 is and has been DEAD for a long time
and the web page was last edited in 2009