As specified below, yo need to use the optical audion in which is aplso connected to the 3.5 mm headphne jack. You need a mic (or mic and conversion box) to inputs SPDIF optical audio input.
http://www.overclock.net/t/1353162/using-a-mic-with-spdif-optical-output
The Audio In port accommodates both digital optical input and analog audio input.
Analog audio line input is accepted through a 3.5mm mini phone jack. The sound input port accepts line-level stereo signals up to 24-bit stereo 44.1-192kHz sampling rate. It also accepts a stereo miniplug-to-RCA cable adapter for connecting stereo equipment to the computer.
Optical audio input is SPDIF format and uses a standard TOSLINK cable with a TOSLINK mini-plug adapter, accepting up to 24-bit stereo and 44.1-96kHz sampling rate.
http://support.apple.com/kb/ht5539