Opensuse 12.2 by default has the virtualbox guest additions inbuilt. But if you want to reinstall then follow the steps.
First install the C compiler and make utilities required to compile the guest additions.
sudo zypper in gcc make
Now install the kernel development packages for the current distro version
Find the kernel type
~> uname -r 3.4.28-2.20-desktop
Update the kernel
~> sudo zypper update kernel-desktop
Now install the kernel development files
~> sudo zypper in kernel-devel
Now reboot suse. After the reboot click Devices > Install Guest Additions in virtualbox window.
Now go to the guest addition cd from a terminal. For example on my system the path is
Or open the pcmanfm file manager and click the CD icon on the left. Go inside the directory from a terminal and run the Linux installer as root.
silver@dhcppc0:/run/media/silver/VBOXADDITIONS_4.2.6_82870> sudo ./VBoxLinuxAdditions.run
If the whole installation process goes fine, then the terminal should show something like this
silver@dhcppc0:/run/media/silver/VBOXADDITIONS_4.2.6_82870> sudo ./VBoxLinuxAdditions.run root's password: Verifying archive integrity... All good. Uncompressing VirtualBox 4.2.6 Guest Additions for Linux.......... VirtualBox Guest Additions installer Removing installed version 4.2.6 of VirtualBox Guest Additions... 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 Starting the VirtualBox Guest Additions done Installing the Window System drivers Installing X.Org Server 1.12 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 silver@dhcppc0:/run/media/silver/VBOXADDITIONS_4.2.6_82870>
It should show done for all entries, mainly the "Building the main Guest Additions module". After the installation completes, reboot suse again.
After reboot suse should take proper screen resolution based on the dimensions of the virtualbox window. "Switch to Seamless Mode" option should be enabled in the View menu of virtualbox window.