%localappdata%\FoundryVTT\Data\modules\plutonium .

Find the latest module .zip via the or their Discord.

You must own the base game files (e.g., via Steam) and point the launcher to that folder.

Are you trying to install the or the Foundry VTT module ? I can give you specific step-by-step instructions for either once I know which one you need. How to install Plutonium Mod