1.  Open a terminal and change to the root user

su -

2.  Then cd into the following directory

cd /etc/yum.repos.d/

3.  Install wget.

yum install wget

4.  Download the repo files with wget.

wget http://download.virtualbox.org/virtualbox/rpm/fedora/virtualbox.repo

5.  Install the dependencies.

yum install binutils gcc make patch libgomp glibc-headers glibc-devel kernel-headers kernel-devel dkms

6.  Update your computer.

yum update

7.  Install VirtualBox.

yum install VirtualBox-4.2

8. Add the VirtualBox user to the vboxusers group

usermod -a -G vboxusers user_name

9.  Reboot your computer.

reboot

10.  Enjoy.  If you run into problems try the following.

/etc/init.d/vboxdrv setup

Note:
I have created a script that will do all this for you, download here.  All you need to do is the following 3 commands.

cd /pathto/fedora-vb-installer.sh
chmod +x fedora-vb-installer.sh
sudo ./fedora-vb-installer.sh