virtualbox.org

When I:
# /mnt/VBoxLinuxAdditions.run

I get:
Warning: unsupported pre-release version of X.Org Server installed. Not installing the X.Org drivers.

Is there a way to force their installation? The version is xorg-x11-server-Xorg-1.15.99.902-5.20140428.fc21.x86_64

if you are trying to run the pre-release version of Fedora 21 which it looks like you are, VirtualBox does not support X.Org 1.15 currently as far as I know. So even if you try and "force it". It won't run.