Installing the VirtualBox Guest Additions is fairly straightforward process but with Fedora 12 you will run into some problems.
Here is the error message you will see when attempting to install Guest Additions.
# /media/VBOXADDITIONS_3.1.6_59338/VBoxLinuxAdditions-amd64.run
Verifying archive integrity... All good.
Uncompressing VirtualBox 3.1.6 Guest Additions for Linux........
VirtualBox Guest Additions installer
Removing installed version of VirtualBox Guest Additions...
Building the VirtualBox Guest Additions kernel modules [FAILED]
(Your system does not seem to be set up to build kernel modules.
Look at /var/log/vboxadd-install.log to find out what went wrong)
Installing the Window System drivers
Installing experimental X.Org Server 1.7 modules [ OK ]
Setting up the Window System to use the Guest Additions [ OK ]
You may need to restart the hal service and the Window System (or just restart
the guest system) to enable the Guest Additions.
To fix this you will need to install the kernel-devel package for your specific kernel. The “uname -r” command will show you the current kernel on your system.
# yum -y install kernel-devel-$(uname -r)
Now you should be able to successfully run the Guest Additions installation script.
# /media/VBOXADDITIONS_3.1.6_59338/VBoxLinuxAdditions-amd64.run
Verifying archive integrity... All good.
Uncompressing VirtualBox 3.1.6 Guest Additions for Linux........
VirtualBox Guest Additions installer
Removing installed version of VirtualBox Guest Additions...
Building the VirtualBox Guest Additions kernel modules
Building the main Guest Additions module [ OK ]
Building the shared folder support module [ OK ]
Building the OpenGL support module [ OK ]
Doing non-kernel setup of the Guest Additions [ OK ]
Starting the VirtualBox Guest Additions [ OK ]
Installing the Window System drivers
Installing experimental X.Org Server 1.7 modules [ OK ]
Setting up the Window System to use the Guest Additions [ OK ]
You may need to restart the hal service and the Window System (or just restart
the guest system) to enable the Guest Additions.