Ao instalar o Guest Additions para Virtual Box no Linux, as vezes, nos esquecemos de instalar os headers do kernel.

CentOS

Se apareceu o erro abaixo

The headers for the current running kernel were not found. If the following
module compilation fails then this could be the reason.
The missing package can be probably installed with
yum install kernel-devel-2.6.32-358.el6.x86_64

Atualize seu sistema

yum update

E instale o kernel-devel

yum install kernel-devel

Aproveite para instalar o gcc

Agora tente reinstalar o GuestAdditions.