1) Maybe you need to install all these required packages
> zypper install gcc gcc-c++ make kernel-source [linux-kernel-headers]
2) This is what I've seen after installing the packages.
> rpm -qa | grep kernel
kernel-default-devel-2.6.31.14-0.4.1.i586
kernel-default-2.6.31.5-0.1.1.i586
kernel-source-2.6.31.14-0.4.1.noarch
linux-kernel-headers-2.6.31-3.4.noarch
3) You have to set this env var so that 'VBoxLinuxAdditions-x86.run' can find all the required files.
> export KERN_DIR=/usr/src/linux-2.6.31.14-0.4-obj/i386/default
4) Mount VBOXADDITIONS iso file. VirtualBox menu 'Devices' -> 'Install Guest Additions...'
5) Run 'VBoxLinuxAdditions-x86.run' as root.
> cd /media/VBOXADDITIONS_3.
> ./VBoxLinuxAdditions-x86.run
If you failed to compile VBoxLinuxAdditions, please refer to the log file '/var/log/vboxadd-install.log' to check what really happened.