The synthesizer has an old floppy disc drive and a MIDI input, but I don't know how to use it with my brother's Mac.
You can get more sounds either by purchasing disks made specifically for the D-20 or you can get MIDI (.mid) or Sysex (.syx) files to send to your keyboard from your computer via MIDI. You can get a copy of the owners manual here. See chapter 5 of the owners manual volume 2 for instructions on how to transfer/receive data via MIDI.
You can buy soundbanks for the Roland D20. For example, try this site. It also includes instructions for how to load them (you don't use the floppy drive, you send them via MIDI from your PC).