How do you insert a CD in the MBK-811A?
    The MK117 and MK220 control pad will allow you to listen to CD music when a CD player is connected to the red audio cable on the shower. This cable is located coming from the electronic control box. Similarly the audio source could be disc player or MP3 player as long as it's connected with the appropriate cable.

To listen you need to connect the device and then choose the audio source on the control pad (CD)

The MBK811a remote has an Audio source button on the left hand side, second from the bottom to select. The volume button will allow control also of the volume.