Just a quick note on getting the guest additions to work on Fedora 20 Alpha. In my setup I was getting the below error when installing VBOXADDITIONS_4.2.18_88780.
... Building the VirtualBox Guest Additions kernel modules The headers for the current running kernel were not found. If the following module compilation fails then this could be the reason. The missing package can be probably installed with yum install kernel-devel-3.11.3-301.fc20.i686+PAE Building the main Guest Additions module [FAILED] ...
As you can see the suggestion above was wrong. After installing the correct package I could install ok.
# yum install kernel-PAE-devel.i686