The A/THR disconnect button is in the levers, actually there is one in each lever. You also disconnect it by retarding the levers to idle.
The push button in the panel is used only in abnormal situations. We never turn the A/THR ON in normal ops. It is automatically armed when we start the take off and it will automatically activate at thrust reduction after take off.
It may be necessary to use it, for instance, if you lose one A/THR channel, after switching to the other AP.