I have a cheap old Aiwa cassette deck connected by a left/right RCA-to-mini plug cable (Radio Shack $3). As soon as you hit "play" on the cassette, the audio signal enters the Mac (I have a 333 iMac, $800). Now, if you have selected "Sound In" from your audio chooser, you'll hear the audio. If you don't hear your music, either you have not told the computer to use the Sound In jack or your cable/tape deck is faulty.
If you do get sound coming in, then you're ready to record. Did your Toast come with Spin Doctor? Spin Doctor is a bit touchy about memory -- turn off virtual memory first. If you have 128 megs or so, you'll be fine. With Spin Doctor open, press play on the tape deck and watch the level meters. You'll see the audio signal light up the LEDs. You're ready to make some files that you can then send to Toast to burn the disc.
I hope this helps.