you need to tell plutoinum where your copy of the black ops 2 is installed.

your game is not installed in the localappdata plutonium folder.

the default location is:

C:\Program Files (x86)\steam\steamapps\common\Call of Duty Black Ops II

if it is not there, then right click on the game in steam and select Properties -> Installed Files -> Browse... and it will show you what folder the game is in.

once you know that, you can use the "game settings" button to update plutonium so it knows where your game files are.