The following picture explains the most common way to connect a SC-55 (or any other SC model) and a MT-32 to a modern PC using a daisy chain approach. With this setup, you press ALL + MUTE on the SC unit to let the MT-32 play. Please note your USB MIDI cable has two plugs labeled on the connector itself. You need to connect the "out" plug to the MIDI IN jack in the SC.

Considering how cheap MIDI USB cables are these days, you can also get two USB cables and hook each Roland unit directly to your PC, like this:

This approach gives you the ability to choose directly from your computer which unit you want to use. Recommended if you want to compose music in addition to retrogame.
USB 1.1 or 2.0?USB to MIDI cables use very little bandwidth. Unless your cable brand calls for it specifically, then it's safe to connect them to an older USB 1.1 hub or computer port. Just mentioning this for people who may have an old USB 1.1 hub lying around.
Do I need drivers for the USB cable?For most cases generic USB to MIDI cables are plug and play and do not require drivers. Some brand name cables may come with a driver disk.
Using a joystick to MIDI cable?Some people still have soundcards with joystick ports on them or are playing with old DOS rigs. Although these cases are outside the scope of this thread, the original daisy chain example can be adapted to fit this particular situation. If you are playing on a modern machine then it's easy. Just replace the USB MIDI cable in the original picture with your joystick cable.
What audio cables do I need?There are many ways to answer this question. Basically you get a set of cables, adapters or combination of both. The MT-32 has two large mono 1/4" phono jacks for left and right channels. For that there are many options out there. These are just a few:


On second generation MT-32 models there is an additional stereo 1/4" phono jack for headphones. You can take advantage of this and use cable like this one. Notice the photo shows an additional phono adapter. You may be able to find a Y splitter cable that has a 1/4" plug already built in.

The cable going from the SC to your PC is an 3.5mm to RCA ("Y" splitter) cable.
Where to get?You can find all your cable needs off places like Ebay or Monoprice.com. If you live in the UK and know of a good online store please let me know so I can add the information here.