You can play recorded midi (see Recording) either on the computer speakers using an installed midi driver, or on an external midi instrument, in that case:

1)  Connect the instrument's midi ports to the computer.  You will probably use a MIDI USB cable to do this.  (Check "midi usb cable" on google to find one).

then,

2)  Select the midi OUT device.  See Selecting the Midi OUT Device.  The play button will turn from disabled to enabled.

                                              play button   --  midi OUT selected

3)  Start the device by clicking on the play button once.

                                               play button -- midi OUT started

4)  Click on the play button again to start playing.

                                               play button -- playing state

To stop playing, click on the play button again.  This reverts to the midi OUT started state.

                                           started state

                                                                 stop button

You can also click on the stop button.  This reverts to the midi  OUT selected but not started state.

                                midi OUT selected but not started state