I'm sorry to tell you this but.
There IS no default.cfg file. At least, I can't find it in the usual location (within the players file).
Just a bunch of encrypted files with names that look like they could contain the .cfg.
But they're encrypted.
sorry I was really hoping to help but it just doesn't seem to be there, I searched the entire folder with windows search and there's no black ops folder in the documents folder so I dont know where it could be...
Ok, ill try to be as descriptive as possible for anyone else having this FRUSTRATING error.
You need to install or copy steam and ALL of its files (except the files in the steamapps folder, this is the content of all your steam games)
My steam is installed on A:\Program Files\Steam\ and this is not the default location, it should be C:\Program Files (x86)\Steam. This is why you will see the default_mp.cfg error, the file is looking in the wrong location.
What I did to fix this is I created a Steam folder in my C:\Program Files (x86) I then copied everything except all my games in the Steam\steamapps from A:\Program Files\Steam\ TO C:\Program Files (x86)\Steam. When I opened steam from the new location (not the shortcut on your desktop, create a new shortcut) I had to run steam.exe as an administrator. Then in the new instance of steam you will see that none of your games are there, I tried copying all the BO2 files over to the steamapps folder but steam didnt detect it, so I just redownloaded the B02 games through the NEW steam client, and when I ran it once it was complete it worked fine.
Im sure there is a less complicated way, maybe just installing a new instance of steam to the default location... but I followed what the guy on a different post said he did and it worked okay. Im not going to screw with it now that it is working.
This works for all of the BO2 games: single player, multiplayer, and zombies