You have 2 options:
Option 1 (old school, PITA): remove the unit. In the back there are 2 RCA Aux input female connectors - one red, one white. Buy and then attach an RCA-male-to-3.5MM-male cable to the unit. Run the wire to your upper cabinet or what have you. Then you can connect it to your laptop.
Option 2 (easy, takes 10 seconds): use the Bluetooth feature. On your Mac go to Bluetooth devices, search for the M303, pair to, enter code 0000 if prompted by your Mac. Voila, done. The M303 will play music from your laptop when you select Bluetooth as the input on the radio.
My vote is for option 2. Actually, my vote is to get rid of that awful clarion system and upgrade, but in lieu of spending $$ go with option 2.
Here is a link to the M303 manual if you want step-by-step instructions on Bluetooth pairing....