If your Mac has a normal 3.5mm input socket - usually called a 'line input', but on some Macs it may be marked as a microphone input - connect a cable with a normal 3.5mm stereo jack plug at each end into (a) the output, or 'headphone', socket of the mini-disc player and (b) the audio input of the Mac.
[..Note that some Macs, e.g; the Mac mini, do NOT have an audio input. In that case, buy a
Griffin 'iMic', or
Macally 'iVoice', or any other USB audio input device which has a 3.5mm input socket, and connect the second end of the stereo lead (b - above) into that..]
Go to System Preferences and then 'Sound', click the 'Input' tab at the top, play a mini-disc at -L-O-W- volume(!) and see if it registers on the 'Input level' volume gauge. If not, -s-l-o-w-l-y- increase the volume - on the mini-disc player and/or the Sound panel 'Input volume' slider - till it shows about two-thirds the way across.
Then go to iMovie, click on the 'Audio' tab at the bottom of the right-hand pane, and see if the audio you're playing from the mini-disc is showing in the volume panel (..moving across from left to right). If so, press the Record button at the right-hand end of the volume indicator!
That will record your music directly into iMovie.
Other - usually free or 'shareware' - programs, such as '
WireTap' or '
Audacity' or '
Sound Studio', let you record from your audio source (..e.g; a mini-disc player..) into your Mac. You can then import these recordings into iTunes. Then import them into iMovie via iTunes ..though iTunes usually encodes them in 'MP3' format, and iMovie will usually play that but prefers 'AIFF' format. You can export from iTunes as 'AIFF' if you specifically choose that ..then import your exported audio into iMovie.