Got RHEL? Got no RHN?
If you have a RHEL box but no access to the RedHat Network for yum updates, you should be able to get them from the DVD ISO that you installed from. Add a new dvd.repoe to /etc/yum.repos.d/. Per RedHat the mediaid= value comes from the .discinfo file located in the root of the DVD. The baseurl path needs 3 slashes, and spaces in the real path replaced by their URL-safe counterpart "%20".
[dvd] mediaid=1269263646.691048 name=DVD for RHEL5 baseurl=file:///media/RHEL_5.5%20x86_64%20DVD/Server enabled=1 gpgcheck=0
Mount the RHEL ISO DVD in VirtualBox with the CD graphic icon on the bottom and it should show up in a GUI UI. If you're using a text UI, you may have to mount it yourself, perhaps something like
mount -t iso9660 /dev/dvd /mnt
If you do this you'll have to change the path above to match '/mnt/Server' instead of the auto-generated path above.
Install kernel dev tools
The OS needs to be able to compile kernel modules for the Vbox guest additions, so it needs a compiler and kernel development libraries. Install it from the yum repo defined above:
yum install kernel-devel gcc
Now mount the image of the VirtualBox Guest additions, using the VBox icon, and mount it on the OS using its built-in automatic thing or manually with the "mount" command. Once mounted, do something like:
And if everything works out you should now have the additions.
Virtual Shared Disk
VirtualShared /VirtualShared vboxsf rw 0 0
# mount /VirtualShared
If you don't want to create the fstab entry, you could do it manually:
# mount -t vboxsf VirtualShared /VirtualShared