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.