How to show mouse pointer location using a mouse click?

Please note first that changing to the left click isn't practical and may conflict with another key binding, like the menus.

But you can use ccsm to initiate another key for showing the mouse location:

If you don't have ccsm: Install From Here

You also need the package compiz-plugins-extra. You can find it in the Ubuntu Software Center.

Go to CompizConfig Settings Manager > Accessibly > Show Mouse:

• Under Use This Plugin left menu: check Enable Show mouse.
• Change the mouse Initiate key binding as below.

To determine the meaning of Compiz button numbers, check this answer.

• Button 1 is generally the Left click
• Button 2 is generally the Middle or Right click

To take effect, you might need to "restart" Compiz:

compiz --replace &

Have a look at this handy tool: Key-Mon

If this is too small or not visible enough and you are not a programmer, ask the author or someone else to change the code here: http://code.google.com/p/key-mon/source/browse/src/keymon/lazy_pixbuf_creator.py

In this similar question there is an explanation about how to modify the image displayed when you click.