You will need a way to get the audio from your stereo to your mac.
Most newer macs do not have an audio line in, relying on USB for input.
The Griffin iMic is a great piece of hardware that has a standard audio line in and a USB out. Go to http://www.griffintechnology.com/imic/ to check it out.
Then you will need a cable to go from your stereo to the imic.
The cable you need depends on the audio out, or headphone jack from your stereo. Some of them are 1/8", while older ones are 1/4".
The iMic line in is 1/8".
Then just go to Radio Shack and get a cable with the appropriate sized jacks. They will have them, and they usually cost in the neighborhood of 6-10 bucks.
Searching the Radio Shack website at http://www.radioshack.com comes up with these products:
a 6 foot 1/8" to 1/8" cable part number Catalog #: 42-2387 for $4.99 and a 6 foot 1/8" to 1/4" cable Catalog number 42-243 for $4.29.
Connect the line out (headphone jack or audio line out) of your stereo/cassette player to the iMic using the cable bought from Radio Shack, then plug the iMic into a USB port on your machine.
Open your System Preferences->Sound/Input and select the iMic as the input source.
Open WireTap Pro, and open the Preferences. Click on the Source tab and set the Source to Microphone/Line Input and the Mic/Line-in to Default Input Device. Set the Record popup to Sound Activated recording. Note: Currently there is a bug in WireTap Pro regarding the Sound Activated option. If this option is selected in the Record popup menu then the Silence Threshold cannot be set to zero. Moving the slider to even a tiny bit to the right is necessary.
Set the file format for the Recording in the Format pane.
Set the saving options in the Saving pane.
Cue your record or cassette but do not start it playing. Then begin WireTap Pro recording using the Record Now button in the WireTap Pro controller. It should start, but not record until audio is detected from the source.
Begin the record or cassette, and WireTap Pro should begin recording as soon as audio is detected. When the record is done, click on the Stop button in the WireTap Pro controller.