1. Devices -> Insert Guest Additions CD image
  2. Mount the cd image (Optional):
    # mount /dev/sr0 /mnt
  3. Install the package requirement (If necessary):
    # zypper in gcc kernel-default-devel
  4. Enable “unsupported modules” to be loaded in the SLES’s kernel:
    Edit /etc/modprobe.d/10-unsupported-modules.conf
    and set: allow_unsupported_modules 1
  5. Run the Virtualbox Guest Additions installation script:
    # cd /mnt
    # ./VBoxLinuxAdditions.run
     Verifying archive integrity... All good.
     Uncompressing VirtualBox 5.0.14 Guest Additions for Linux............
     VirtualBox Guest Additions installer
     Copying additional installer modules ...
     Installing additional modules ...
     Removing existing VirtualBox non-DKMS kernel modules done
     Building the VirtualBox Guest Additions kernel modules
     Building the main Guest Additions module done
     Building the shared folder support module done
     Building the OpenGL support module done
     Doing non-kernel setup of the Guest Additions done
     You should restart your guest to make sure the new modules are actually used
    Installing the Window System drivers
     Installing X.Org Server 1.15 modules done
     Setting up the Window System to use the Guest Additions done
     You may need to restart the hal service and the Window System (or just restart
     the guest system) to enable the Guest Additions.
    Installing graphics libraries and desktop services components done
  6. You can now reboot your system and check the if the new modules are used:
    # reboot

    and then after the reboot:

    # lsmod | grep vbox
     vboxsf 47920 0
     vboxvideo 12669 1
     drm 322623 2 vboxvideo
     vboxguest 298126 3 vboxsf

Ref: vmoutoussamy.blogspot.com.tr