You don't need to buy RPG Maker to play RPG Maker games, or install mods for them.
Lust Doll+ is free with an option to support it's Dev.
If you want to install a mod, you shouldn't need RPG Maker, unless you're installing conflicting mods.
But if you want to create a mod, then yes, you do need a copy of the RPG Maker used to create the game, and the game's project files.
Bringing this up since OP didn't clarify if they were installing or creating a mod.