It has been far too long for me to remember. The data is stored on tape in audio format, so it's just a matter of connecting the audio between tape recorder and the Commodore 64. Here is a link to the documentation:
Commodore 64 documentation
Scroll to the bottom of the web page for the doc downloads.
I'm looking through the manuals and will post more as I discover it.
 Ok, after searching around on the net and some Commodore 64 reading, I have to admit it's not as easy as I thought. I'm not giving up, I'll let you know if I run across a feasible solution.