Many MiniDiscs do NOT have a digital output - you can record through the analog stereo line or mic inputs any stereo source that matches the proper levels (these levels vary depending on which Mac)
In order to record digitally, you have to have a digital source with a TOSLINK optical output, and a Mac that has an Optical input. Otherwise, you can buy USB and FIrewire interfaces with ADAT optical, S/PDIF stereo inputs, Tascam TDIF 8 track digital connectors and AES/EBU connectors, depending on the source of the signal and how much money you can spend.