I make heavy use of this and it doesn't have separate buttons as such,
they are just a hidden part of the mouse pad nearest you, a clicky
part. I have always found them difficult to use, because guessing where
to actually press and they need a lot of pressure, much more pressure
as you move out either side from close to the middle.

A;most impossible to just press and click in the right spot, without
looking where you were pressing, as there is no tactile feedback.

Last night, I superglued a short bit of 1mm CSA copper wire on the pad,
to identify the mid-point between the buttons - so much better.