1. Configure yum to use the RPMFusion repository:
su -c 'yum localinstall --nogpgcheck http://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-$(rpm -E %fedora).noarch.rpm'
2. Install guest additions:
Fedora19: sudo yum install VirtualBox-guest-4.2.14-1.fc19
Fedora20: sudo yum install VirtualBox-guest-4.3.14-1.fc20