A new option has been added in the OSD menu of this core called "UserIO Joystick" that allows playing with joysticks of Mega Drive/Genesis (DB9) or Neo-Geo/Supergun (DB15). This new feature is disabled by default, therefore you must enter the menu (F12) the first time to activate it and save the config if you want to keep it on for the next time.
You can choose native original gamepads (SMS) or non original (megadrive). On non original option button maping is A=1/Start, B=2.
For controlling everything with the UserIO Joysticks (including the OSD menu), is also needed to update the files: MiSTer and menu.rbf from the root of the SD card. The link to these files is here (download the latest release):
MiSTer_Main: https://github.com/yavimaya/Main_MiSTer/tree/master/releases
Core Menu: https://github.com/yavimaya/Menu_MiSTer/tree/master/releases
Menu control from DB9 joystick: Start+C-> Show OSD menu | A-> Enter | B-> Esc
Also you can get more information about DB9 fork from this link: https://github.com/antoniovillena/MiSTer_DB9.git