Open up a Super User Terminal Mode (from the Applications menu | System Tools | Terminal Mode – Super User)

First, we need to add the repository where all the cool backports and upgrades are. At the command prompt, type:

sudo nano /etc/apt/sources.list

and add this line at the bottom:

deb http://www.backports.org/debian lenny-backports main contrib non-free

To save, use CTRL – X and Y to save.

Now we need to add the public key ring for backports, at the command prompt, type:

wget -O – http://backports.org/debian/archive.key | apt-key add -

Once that is successful, update apt by entering:

apt-get update

Finally, we’re ready to install Open Office 3 on Debian Lenny. Type this at the command line:

apt-get -t lenny-backports install openoffice.org

And it will show dependancies and what files it will need to download and install. If all looks ok, enter Y and the install will commence.

Got my info from a few sources here and here.