To find the disabled devices in OSX
3.5 mm ports on Macbook Airs don't support regular 3.5mm audio in plugs. Their 3.5 mm ports only support "three ring" plugs. How to use an external microphone with a MacBook Air

The 3.5 mm audio port on Macbook Pros can do regular audio in and audio out. But they default to out and needs to be changed to audio in using the "Use audio port for:" option from the Input tab of the Audio preference pane in System Preferences.



More information here: MacBook and MacBook Pro (13-inch): How to use the combination audio port for an audio input device