Create a new macro and set the trigger for it to be USB Device Key Trigger
The macro will then look like this
Then just click your mouse button to enter it into the trigger field.
I can't test this myself since I have a Magic Trackpad, but a quick search of this forum leads me to believe it should allow you to trigger with a mouse click as required. Your macro will then have to determine what it is that you've actually clicked on and that might be a bit tricky to work out.