There are several guides if you Google for it. This Ubuntu guide will work.
As for the location of your VDI: start VirtualBox, and check the properties of your virtual machine and/or check the available media from the menu.
From the help pages:
Starting with version 4.0, by default, each virtual machine has one
directory on your host computer where all the files of that machine
are stored -- the XML settings file (with a .vbox file extension) and
its disk images.
By default, this "machine folder" is placed in a common folder called
"VirtualBox VMs", which VirtualBox creates in the current system
user's home directory. The location of this home directory depends on
the conventions of the host operating system:
On Windows, this is %HOMEDRIVE%%HOMEPATH%; typically something like
C:\Documents and Settings\Username.
On Mac OS X, this is /Users/
username.
On Linux and Solaris, this is /home/username.