You click by pressing down. Just as you would on the track-pad of a mac laptop.
There are tools to modify the preferences and add short-cuts, but I shouldn’t need to, in my humble opinion. And it seems like I might not. It seems like there are multiple solutions.
I like the idea presented by whiterabbitdesigncompany. I am a photographer by trade so spend a lot of time on a Wacom and on the trackpad and am comfortable with that, so having a piece of hardware I can put away when I am not using Sketchup is a great solution.