Oracle VirtualBox

How to install the extension pack.
1. Open the VirtualBox application.
2. Go to Preferences.
3. Click the "Extensions" icon.
4. Add the extension pack by clicking the little diamond.

How to install from the commandline.
VBoxManage extpack install <tarball> |
                   uninstall [--force] <name> |
                   cleanup
Source

Failed to open the hard disk.
Cannot register the hard disk  becuase a hard disk  with UUID  already 
exists. I looked all over the place to find what was causing it and
finally found a fix. Of course it requires the command prompt, so open 
that junk up and let’s get started!
**Note that I am on VirtualBox version 4.1.6 and from what I’ve read this 
command used to be something different a few versions ago. I think instead 
of setdhuuid it was setvdiuuid, but don’t hold me to that.**
Open the Command Prompt
Change the directory to where VirtualBox is installed 
(Default: C:\Program Files\Oracle\VirtualBox). Type the following: 
VBOXMANAGE.EXE internalcommands sethduuid pathtovirtualharddisk
Note that internalcommands and sethduuid need to be lowercase.  I spent more 
than a couple minutes trying to figure out why things weren’t working.  :)

Source