You can record into an application like Garage band then transfer the files to iTunes, However the best thing you can do is connect the line out from your record player to the Mic input of your mac then you can use a free piece of software like Audacity http://audacity.sourceforge.net/
This is a recording interface that will allow you to record the input from your tapes then edit your tracks if needs be and save them into an MP3 format which you can then easily add to iTunes by clicking File then add file to library.
Take a look and let me know what you think or if you have more questions.
You can use the iMic if you prefer as the Audacity software will allow you to use any recording source, So if you wish to use th iMac all you would do is select it from within Audacity and then reord your songs from your Vinyl.
Personally I have never found a need to use a third party recording interface as when I record my vinyl all I use is a standard headphone to Mic cable but the iMic adapter will work just fine.
Let me know if you need more info or clarification.ThanksEric