When clicking "install VBox guest additions" from the VBox manager VM context menu on a Windows guest, the Windows installer launches automatically, but things are not as simple on a Linux guest:
1a) From the VBox manager "Devices" menu, click "CD/DVD Devices" -> "Choose a virtual CD/DVD disk file" and navigate to /usr/share/virtualbox and select VBoxGuestAdditions.iso (Note: in Archlinux the location of the the guest additions iso is /usr/lib/virtualbox/additions/VBoxGuestAdditions.iso and is provided by the package virtualbox-guest-iso)
1b) Alternatively, you can click "Insert Guest Additions CD Image" from the VBox Manager "Devices" menu.
2. mount the cd-rom/dvd drive onto a mountpoint. To find the name of the cd-rom you can do
which (depending on your machine) will reveal /dev/cdrom or /dev/sr0
sudo mount -o loop,ro /dev/cdrom /mnt/test
3. cd into the mountpoint and run the install script ending in extension .run (don't run the other scripts ending in .sh) as root:
4. After rebooting the VM, you should find that mouse pointer integration, full-screen mode, shared clipboard, folder sharing between host and guest machines and other useful features now work.