Make sure to install them on the 2 game apps on Steam (Campaign/Zombies, Multiplayer) and that both of them are installed. Also, check that the folder you selected in the Plutonium launcher is actually the folder which Steam is currently using for the game by comparing the paths.
For checking the selected folder in the Plutonium launcher, follow this:
Press Win+R, paste this into the window that opens: %localappdata%\Plutonium, press Ok, open config.json, then check the path set for "t5Path".
For opening the folder Steam uses for the mode: right-click the mode from your Steam library > Manage > Browse local files.