You will need to be the root user for the following tasks. Login to a root shell or "su -" in a terminal window.
The installation of VB will require the building of kernel modules. If DKMS (Dynamic Kernel Module Support) is installed it will be used and will simplify kernel upgrades. Installing DKMS from the EPEL repository is recommended before installing VirtualBox. Don't forget to configure the yum-priorities plugin. Installing DKMS may pull in required development dependencies, depending on the package source.
yum install dkms
If DKMS is not used the Guest Additions will need to be reinstalled after every kernel update.
If the development environment and kernel source are not already installed:
yum groupinstall "Development Tools"yum install kernel-devel
You may also choose to only install a minimum set of individual development tool packages (at least gcc and make are required, in addition to kernel-devel) rather than the groupinstall which some may consider overkill. Replace "kernel-devel" with "kernel-PAE-devel" if using a PAE kernel on CentOS-5. If you are not using a standard CentOS kernel, you must acquire and install the source for your kernel from wherever you got the kernel. Do not try to install a Xen kernel in a Guest OS, or run KVM or any other nested virtualization hypervisor.
From the VM Devices menu select Install Guest Additions.... If running a GUI the virtual CD image for guest additions will automount - for instance under CentOS-6 with KDE it is mounted under /media/VBOXADDITIONS_5.0.14_105127. If autorun is enabled in a GUI you may get a window asking if you want to let the autorun execute, and asking for root authorization. Take all the defaults and the install should complete.
If not in a GUI, or if there is no autorun, as root mount /dev/cdrom (if necessary) and cd to the mountpoint. Type ./VBoxLinuxAdditions.run to install.
