I am trying to connect an audio device (analog synthesizer) to the input of my 13-inch Mid-2012 Macbook Pro running OS X Mavericks 10.9.3. The synthesizer's output is a standard 1/8th inch stereo plug. According to http://support.apple.com/kb/ht3625, I should be able to switch this from System Preferences. However, my screen looks like this even when my device is plugged in:
There is no option to use the audio jack as an input.
How do I connect my device to the audio input?