Getting CentOS kernel sources to build VirtualBox kernel module

Building the VirtualBox Guest Additions kernel modules     [FAILED]
(Your system does not seem to be set up to build kernel modules.
Look at /var/log/vboxadd-install.log to find out what went wrong)

/var/log/vboxadd-install.log:
Makefile:23: *** Error: unable to find the sources of your current Linux kernel. Specify KERN_DIR=<directory> and run Make again..  Stop.


method:
as root

# yum groupinstall "Development Tools"
# yum groupinstall "Development Libraries"
# yum install unifdef rpm-build

export KERN_DIR=/usr/src/kernels/2.6.18-164.15.1.el5-i686/

sh ./VBoxLinuxAdditions-x86.run