I bought [021501 MRC Loco Genie (steam heavy)].
I'd like to plug in the my steam.
The instructions say "if your loco has a 9 pin JST plug or NMRA 8 pin socket, you can simply unplug the original connector and plug in the decoder."
What kind of decoder does it fit?
I would like you to tell me the specific product name.
Not the best forum for this issue.
Use the Model Railroader DCC electronics forum. A few forums down. Very easy.
The General Discussion forum under Model Railroader shows you how to upload pictures which can be helpful.