As mentioned in the section called “Some terminology”, the Guest Additions are designed to be installed inside a virtual machine after the guest operating system has been installed. They consist of device drivers and system applications that optimize the guest operating system for better performance and usability. Please see the section called “Supported guest operating systems” for details on what guest operating systems are fully supported with Guest Additions by VirtualBox.
The VirtualBox Guest Additions for all supported guest operating systems are provided as a single CD-ROM image file which is called
VBoxGuestAdditions.iso. This image file is located in the installation directory of VirtualBox. To install the Guest Additions for a particular VM, you mount this ISO file in your VM as a virtual CD-ROM and install from there.
Each version of VirtualBox, even minor releases, ship with their own version of the Guest Additions. While the interfaces through which the VirtualBox core communicates with the Guest Additions are kept stable so that Guest Additions already installed in a VM should continue to work when VirtualBox is upgraded on the host, for best results, it is recommended to keep the Guest Additions at the same version.
there is tips for installing guest addition for linux :