Installing VirtualBox in Linux could fail about no kernel sources available DKMS.

If this is the case then set KERN_DIR variable and then again retry installing VirtualBox

## Current running kernel on Fedora ##

KERN_DIR=/usr/src/kernels/`uname -r`

## Current running kernel on CentOS and Red Hat (RHEL) ##

KERN_DIR=/usr/src/kernels/`uname -r`-`uname -m`

## Fedora example ##

KERN_DIR=/usr/src/kernels/2.6.33.5-124.fc13.i686

## CentOS and Red Hat (RHEL) example ##

KERN_DIR=/usr/src/kernels/2.6.18-194.11.1.el5-x86_64

## Export KERN_DIR ##

export KERN_DIR

Source: http://www.if-not-true-then-false.com/2010/install-virtualbox-with-yum-on-fedora-centos-red-hat-rhel/