From the gnome desktop, go to:
system->administration->synaptic package manager
select virtualbox-3.0, then click "apply"

Or from the command line, type "sudo apt-get install virtualbox-3.0"

Download the deb file and double click on it. Then you select install.

The VirtualBox in the repositories (OSE) is not full featured. To get all of
the features (such as usb support) you need to full or PUEL version from the
VirtualBox website. Download the full version which is free to individuals.
(See: http://www.virtualbox.org/wiki/Editions)

http://www.virtualbox.org/wiki/Linux_Downloads

Choose the right architecture for your distribution. For Ubuntu it will be
for either 9.04, 8.10 or 8.04 and AMD64 or i386. The file will be a .deb and
you can install it by clicking on it.

Here is a link to a tutorial on setting up the /etc/apt/sources.list file, with
screen shots and step by step instructions.

Installing VirtualBox 3.0 On An Ubuntu 9.04 Desktop
http://www.unix-tutorials.com/go.php?id=4134

Just download the Virtualbox in deb extension. Save it a common folder
e.g downloads and then right click it with your mouse and chose the deb
installer that comes up in the window and away you go