After installing Centos 5.7 as a Virtualbox 4.16 guest OS the following steps should install the Guest Additions.

1. With root privileges in a terminal yum install gcc kernel kernel-devel -y
2. Click Install Guest Additions under Devices in the main guest window which mounts the folder at /media/
3. cd into the mounted folder and then as root run ./VBoxLinuxAdditions.run
4. Check the terminal output as the install takes place and if there are no errors reboot the Centos Guest OS
5. When the Centos guest OS restarts login then minimise and maximise the main guest OS window and the Centos guest screen resolution should automatically resize to your actual screen resolution minus the guest os window tool-bars, unless you select full screen output for the guest.