[Device > Install Guest Additions] to download guest OS additions zip. Then exact to a folder.

In that folder, run:

./VBoxLinuxAdditions.run If you got an error on “build main” section with a message about KERN_DIR. Try: yum install -y gcc kernel-devel kernel-headers yum install -y kernel sources # I’m not sure if those are necessary. but i’ve installed it. And reboot client OS.

And run again: ./VBoxLinuxAdditions.run Now you should get all “OK”. Reboot client OS again. Now the additions should take placed.

If still not work, this is further problem shooting teach: How to install VBox Guest Additions on CentOS 5.6-No Problem