First step: Install the base system
$ sudo pacman-s--needed dkms qt VirtualBox virtualbox-guest-iso
$ sudo gpasswd-a ' whoami ' vboxusers
$ sudo systemctl enable dkms
$ sudo vim/etc/modules-load.d/virtualbox.conf
Vboxdrv
Vboxnetflt
Step Two: Install the expansion pack
$ wget Http://download.virtualbox.org/virtualbox/4.2.0/Oracle_VM_VirtualBox_Extension_Pack-4.2.0-80737.vbox-extpack
$ sudo virtualbox
Use the above command to run VirtualBox under root, click on the "Manage with Global Settings" menu item, click on the "Extensions" button in the Pop-up dialog box, then click on the "Add Package" icon on the right, select the extension package you just downloaded, install it.
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Arch Linux Installation Virtualbox 4.2.0 Memo