You need a DB15 to MIDI adapter. Something like:
https://www.rapidpcs.co.uk/store/11/56/product/1428/
One end goes into your (Soundblaster?) joystick port and the MIDI OUT cable goes to the MIDI IN of your SC-55. You then either need to plug some headphones/speakers into the SC-55, or connect the left/right phono OUT of the SC-55 to the line-IN of your soundcard (to mix the SC-55 audio with that of your soundcard - MIDI cables do not transmit sound).
The better long-term, more-flexible option is an external desktop mixer, that you connect your soundcard, SC-55 and anything else you want to use, and THEN to your speakers/headphones.
Be aware that the Soundblaster cards are not 100% compatible with all games featuring MIDI soundtracks. They have various bugs. The use of something like SoftMPU can mitigate this to an extent for some titles, but not entirely.
Sorry.... legacy PC audio is not entirely a simple topic to summarise!
My collection database and technical wiki:
https://www.target-earth.net