Make sure you followed the instructions in the install guide and got the files from there: https://plutonium.pw/docs/install/
Then open the launcher, go to the tab for your game and click Setup (or Game settings if it says Play) and select the folder that contains all the game files, for example pluto_t6_full_game
If you get an error saying you didn't select the correct folder then select the correct folder.
Don't select a folder that contains another folder select the folder that has all your game files
This folder should have multiple folders and files inside related to your game.