The problem is that opevz support needs to be enabled at compile time and most of the htop rpms I've seen don't have it. You'll need to either rebuild the rpm and change the switch or build from source.
wget downloads.sourceforge.net/project/htop/htop/1.0.1/htop-1.0.1 .tar.gz
yum install ncurses-devel gcc make
tar xvzf htop-1.0.1.tar.gz
mv /usr/local/bin/htop /usr/bin/