Analog tape into Garageband ver 4

iMac 27 inch late 2013 running 10.11.2. Garageband ver 4.1.2 (248.7) Seems to be no current answers in here so here we go.

Trying to get some old reel to reel audio tapes into garageband to digitize the contents. Now my Apple Help tells me that the headset jack can be used for either input or output of audio sources. All I have to do is change it in the Sounds pref. Well sound pref give me no such option when I'm plugged into the iMac with the audio deck. All that shows up is the built in mic of the iMac. Feeding the line output, which is the tape decks only output. Cable I'm using is a male RCA plugs (they still call them that ?) with a mini heads plug.