Do you have the option of installing the 18+ DLC from Steam directly? If it's available in your region, that would be the easiest option. Steam should automatically put the file in the right place.
If not, it should be a matter of finding the game in your steam library, right clicking on it and going into Manage -> Browse local files. Then open the "game" folder and copy the .rpa file into that folder. The game needs to be restarted after that (if it's currently running).
Also, make sure you downloaded the patch from https://zetsubou.games/tomboys-need-love-too/index.html
If you downloaded it from somewhere else, it's possible that it's an old version of the patch made for a previous version of the game.