Proper multi-monitor support isn't available in the version of Xfce that comes with 12.04, so you'll have to use arandr. Look for it in the software centre or, from a terminal window:
Once installed, you'll find it in the Settings Manager. Double-click to start it up. Drag the monitors to position them how you would like them then Layout->Apply.
sudo apt-get install arandr
To make it permanent (so you don't have to run arandr everytime you login):
1. Layout -> Save As (select a name, default save directory is ~/.screenlayout)
2. Create a startup entry (Settings Manager -> Session and Startup -> Application Autostart) where the command will be:
...where <USER> is your username and <FILE> is the filename you gave in the previous step.