Log in as root and type in terminal:

• yum install gcc -y
• yum install kernel sources -y
• yum install kernel-devel -y

Restart the system.
Mount the VboxGuestAdditions.iso.
Move to the following directory:

• cd /media/VBOXADDTIONS_2.2.4_47978

As root execute:

• sh ./VBoxLinuxAddtions-x86.run

Restart the system. Done.

as being mentioned here.