TL;DR at bottom
First, decide whether you want the normal download which is fully packaged and ready to go, but will take significantly longer, or torrent download which will be much faster, but require merging folders and knowledge of torrenting software. Start downloading Plutonium and the game files which we've packaged all together here or grab this zip and start both torrents, save plutonium.exe for later. Regardless of your choice, this will probably take an hour or more to download and, depending on CPU and storage performance, half as long to unpackage. So get that started before moving on.
While that's going, you're gonna have to create a Plutonium account. This is because it uses its own servers and forum site to handle friends, avatars, rank, etc. Go ahead and do that here.
There's a good chance you won't need this, and you can probably just skip this step, but in case you get any missing DLL errors when starting the game up, you can download the appropriate DirectX files from Microsoft here.
Your download is probably still going, go play some games or something and come back when that's done. Done? Okay, now we're going to want to make a folder somewhere. Anywhere is good EXCEPT protected directories. So no Program Files, Desktop, User folders, etc. Find a good spot, make a new a folder and call it "Black Ops 2" or "COD BlOps2" or whatever you like. The official installer uses "Call of Duty Black Ops 2"
Next we're going to take that archive called CoD_BlOps2_Plutonium.7z we downloaded and extract it directly into the folder you created. If you took the torrent route, extract the base game zip into the folder you created. Then, extract the DLC archive in the same folder and merge them, you should overwrite anything from the base game files with the DLC files. Next, put plutonium.exe in the folder as well. Make sure whatever you use to extract doesn't create a subfolder or it's going to take you forever to move the files again. Get this right. This is going to take a while again, as the files are basically as compressed as a black hole, but we're almost there!
Once you've got that extracted, you should have that executable named plutonium.exe, start that. After a few seconds, a launcher should appear and you can login with the account you made earlier. Aftwards, you'll see several games available. You don't have any of the files for the other games, but if you want to use Plutonium for any other CoD games in the future, you can move this EXE to wherever is convenient for you as long as you don't move the rest of the files.
Select "T6 Multiplayer," then select the "Setup" button. Click "Select Dir" and then select the same folder we've put all the files in.
That's it! If all went well, you can click Play, go into the server browser and search "Pellux" to join!