When you get to the boot menu from the CentOS iso, make sure the first option "Install or upgrade an existing system" is highlighted, then press the TAB key. You can now edit the boot options. The line looks something like this:
Append you ks= line to that, so it looks like this:
vmlinuz initrd=initrd.img ks=nfs:192.168.xxx.xxx:/export/install/ks.config
Then press ENTER to boot. That should do the trick for you.