At the bottom left corner it says Add a Game, click that
In the context menu that appears, click Add a Non-Steam Game...
Click Browse... in the bottom left of the menu that appears.
Navigate to the plutonium launcher that you downloaded from the website and double click it.
Back on steam, click Add Selected Programs
Verify your files in Steam, make sure you did not download any random dlls of the internet and repair the dependencies. This error usually just indicates that a binary file can't be loaded since it doesn't "fit" your system.