You can use the microphone in any Performance or SONG/PATTERN. When on the main PLAY screen of any of these modes you can set the microphone up by pressing F2 AD. Here you will find all the A/D parameters, most important of which is the MONO/STEREO parameter. You want to set it L for Left input. That will do it. Now you can use the PAN to move it if you want to! Ad effects other than the default reverb, etc.

Of course, there is one other setting for the A/D input that is found in UTILITY, it is the F2 button I/O - this is where you select MIC or LINE input.