Free software to convert audio cassettes on my Mac ?
 

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Please ask all tech questions in the forums. Thanks!

And welcome to the site. :)

Your Mac undoubtedly has an audio input, so the hardware issue is resolved.
Now you're down to a software issue.

Yes, there are free options for recording/editing audio. One program in particular is quite decent at it: Audacity.
In fact, the program has been ported to Windows, Mac OS9 and Mac OS X, and Linux.

Download Audacity for Mac: http://audacity.sourceforge.net/download/mac

Give that a try.
And please do post back, letting us know how you like it. Come share your success story.
Or ask further questions on audio editing, if needed.

Hope that helps. :)