On my previous tutorial, I've show you how to install a headless VirtualBox on a ClearOS 6.3. Now, on this tutorial, I will show you how to install the extension pack. This extension pack is required to enable the remote administration of Virtualbox using RDP or phpvirtualbox.
Steps to install Virtualbox Extension Pack
cd /tmp
wget http://download.virtualbox.org/virtualbox/4.2.4/Oracle_VM_VirtualBox_Extension_Pack-4.2.4-81684.vbox-extpack
VBoxManage extpack install Oracle_VM_VirtualBox_Extension_Pack-4.2.4-81684.vbox-extpack
Make sure you execute the command above using root privilege.