Zoom is normally mouse scroll wheel up, so depending on the touchpad you might be able to 'Two finger' swipe upwards (Place two fingers on the touchpad and move them upwards) or you might be able to use the right hand side of the trackpad (Place one finger on the far right hand side of the trackpad and move upwards)
OR there could be a third button in the middle of your normal left and right click which you can 'Hold' and swipe with.
Sometimes these options are disabled in game, if you have a mouse handy you'd be better playing with that!