Here is how you can install VirtualBox in Fedora 15. Open up the terminal and type in the following commands (make sure you have enabled sudo for your user):
wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | rpm --import - sudo wget http://download.virtualbox.org/virtualbox/rpm/fedora/virtualbox.repo --output-document=/etc/yum.repos.d/virtualbox.repo sudo yum install VirtualBox-4.0 dkms
Hope this helps.