On the 900 series, which are equipped with a multi-touch touchpad, you can perform a middle mouse button click by tapping on the touch pad with two fingers simultaneously or by clicking both Left and Right buttons at the same time.
On the 700 series, only the left/right button comnination works by default. However, it is possible to tweak the touchpad settings such that hitting it in the bottom left corner is a middle click, and hitting it in the top left corner is a right click.
To do this, you have to edit the Synaptics driver settings in /etc/X11/xorg.conf, the configuration file for the X window server.
In a terminal/console, first make a backup of the configuration file:
sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.orig
Then edit the X window server configuration
sudo kwrite /etc/X11/xorg.conf
Search for the section
Section "InputDevice"
Identifier "synaptics"
Driver "synaptics"
Change or add the following lines:
Option "LeftEdge" "1500"
Option "TopEdge" "1500"
Option "BottomEdge" "4800"
Option "HorizScrollDelta" "0"
Option "VerticalScrollDelta" "200"
Option "LTCornerButton" "3"
Option "LBCornerButton" "2"
Type man synaptics from the console to see what these options mean.
While you're at it, you may also wish to enable switching to a text-mode console by commenting out the following line by placing a # sign at the beginning:
# Option “DontVTSwitch” “true”
This will enable you to login in a text-mode screen by pressing Ctrl-Alt-F2. To go back to the desktop screen, use Ctrl-Alt-F3. This may be handy if you mess up the X server settings.
After saving the changes, check for errors by typing from the command line:
sudo X -probeonly :1
If your changes are correct, the screen will flash a couple of times during 20 seconds or so. Detailed information will be in /var/log/Xorg.1.log in case you're interested. However, if you made a fatal mistake, you will immediately get an error message, in which case you have to fix that before you continue - or you won't be able to login anymore.
Now logout and it should work. If it doesn't work because you have thick fingers, you can consider increasing LeftEdge and TopEdge, and decrease BottomEdge by 200 or so.