Button class implements two states of button, making your buttons more responsive :
local button_pushed = Bitmap.new(Texture.new("images/start_down.png"))
local startButton = Button.new(button_default, button_pushed)
local x = ((application:getDeviceWidth()-startButton:getWidth())/2)
local y = ((application:getDeviceHeight()-startButton:getHeight())-20)
startButton:setPosition(x, y)
stage:addChild(startButton)
startButton:addEventListener("click",
function()
//do what you want on click
end
)