Caiolechu what were you trying when you tried to do custom games?

In options then graphics there's options for fullscreen and no border so you can enable/ disable those

If you mean the actual plutonium files and not the game folders then do win+R then type %localappdata% then you should see plutonium on the list but if you meant in the launcher to set up the game then you might have saved the game files in the wrong place