i'm having the same problem and getting the same message, despite the fact that i just downloaded the update and can see my friends playing online right now. activision is very disappointing with this...
i restarted, checked for corrupted files in downloads, reset router and modem, tried to join a friend's session in progress, nothing has let me on multiplayer.
i had similar issues with the first Black Ops when it came out. I think i'm done with activision, since they create headaches, sell them for top dollar, then don't seem to care enough to fix them or make a fix easily accessible to the consumers who've loyally purchased more of their product.
According to Activision Assist, they are aware of the issue and are looking into it as we speak. Thank you for your patience.
CODBlackIce | Global Forum Moderator
Black Ops, Modern Warfare 3, & Black Ops II