Installing Virtual Box guest additions on CentOS5 and Scientific Linux 5 (x86)

First step have the proper libraries installed

yum install gcc-c++ kernel-devel kernel-headers

Next navigate to the folder where your kernel source is located

export KERN_DIR=/usr/src/kernels/bla bla bla

cd into the VBox guest additions folder then run

sh VBoxLinuxAdditions-x86.run

Enjoy, you are done :)