Connected device isn't showing listed on Sound Input?

Ah' OK. If you have a > https://support.apple.com/kb/sp665?locale=en_US then you are correct and it only has a Headphone jack.

When your are shopping for a USB audio adapter, you need to make sure that is has both input and output and then plug the recorder into mic or input port of the USB adapter.

Connect the audio cable to the output (slot) jack of the tape recorder.

Make sure that the audio cable is firmly seated in the input (microphone) jack of the Mac Mini.

Then if the Mac Mini is still not see the sound input in the Sound Preference, you may need to try a different audio cable.

Hmm.... what year model is the iMac?

If it is a 2015 or earlier, open the Sound Preference and select the Input tab. It should show internal mic with nothing plugged in and then switch to the external input when the audio cable is plugged in. If it does not then there is a good chance that the micro switch inside of the input port is bad and not switching the audio to external input. To over come this problem, you can use a USB audio adapter instead of having the iMac’s input port repaired.

If it is a 2017 or later with a single combination Headphone/Input port, then it will not work and you need to use a USB audio adapter.

for example > https://www.amazon.com/Sabrent-External-Adapter-Windows-AU-MMSA/dp/B00IRVQ0F8

If it only has one audio port, then it is not a 2012 iMac. The iMac models that only have one audio port only work with combination headsets for input.

You need to get and use a USB to audio adapter like the one from amazon that I posted above.

Double check to make sure that your plugging the recorder into the Input line-in jack and not the Headphone line-out jack.

You're welcome.

There are other methods for getting those tapes into your Mac. The easiest one, is to take or send them out and have them put onto a flash drive or hard drive.