I assume you have a cassette player already attached to a stereo receiver. The cord you bought with the mini pin (headphone) jack on one and RCA plugs on the other...would fit like this...RCA jacks to the recording output section of your stereo receiver and the headphone jack to the line in port on your sound card. If you have a soundcard that has
RCA inputs it should also have a line in port that would accept that mini pin connection. Some hi-end soundcards have normal size headphone type jacks but a simple adapter would allow the connection. If you do not have a stereo receiver....try using a cord that has mini pin (headphone) connections on both ends and go from the headphone port on portable cassette player to line in port on the soundcard.