On my laptop I was able to do this as follows:
Go to Dash Home and type Displays and select the Displays control panel.
Plug in the external monitor.
Turn off mirroring. You should see two screens depicted in the control panel display.
Select the external monitor's box, move it to the orientation you would like, turn the external monitor on with the panel's on/off control, set its resolution, and then press Apply. They should both blank for a second or two and then come on.
I use Unity 3d and was unable to put them side by side as the sum of the two widths exceeded 2048 (my Laptop is pretty wide compared to how tall it is.) I simply positioned the second display under the first, set a resolution for it and turned it on (with mirroring off, of course), and it came up properly as above.
I was able to move back and forth between screens by moving my cursor down off my laptop desktop and onto the external monitor desktop. Just drag the presentation or other window down to put it on the external monitor.
I suppose I could have also switched to Unity 2d instead of putting one screen under the other in the virtual desktop, but I don't see what difference it makes.
The command line program to do this sort of thing is xrandr. I've used that command to put together a little shell script I would it run when I connected to an external monitor to set the second screens's resolution properly and activate it. I put a shortcut on my desktop for it so I wouldn't have to make my audience wait. The particular xrandr options are --right-of --left-of --above and --below.
xrandr --output VGA-0 --mode 1280x1024
xrandr --output VGA-0 --below LVDS
xrandr --output LVDS --mode 1440x900