I now have 2 pc's in the same room, mine and my sons. I have 2 steam accts, 2 CoDWaW games, 2 map managers with downloaded MODS.  I log both accts in, click mods, click co-op, click host LAN and tried host online, invite my son or vice versa and get this msg. "Trying to join potential matches." After 20 secs or so I get this msg. "This game is no longer available." I reset the router as 1 person suggested and still nada. Any ideas of what might be wrong?

Why would you try and host via LAN if you intend to play online?
You need to go to "Host online" if you want to host online.
Or are you connected via LAN? Sorry if I got your question wrong but it sounds like you mixed up these two.

Great, I got it to work by hosting online and changing max ping and to zombie mode. Played 1 game and now when I try to launch a game it says exceeded 400 fx something error and boots me. Now how do I fix that lol?

I had this problem, I fixed it by getting new internet. My old one was too slow.

I did this with a buddy once. You can try connecting your pc's to one another via ethernet cable and play over lan that way. Unless you are trying to host online and have other join you guys then I'm not sure why you arent able to connect with one another.

This error occurs randomly sometimes. But if it persists you could check your mods-folder for any loose files, they'd get loaded too with every mod.

What he^ said and...
I had same issues before. I had really shitty internet back then too. This link helped me:

http://www.tomshardware.com/forum/113321-13-coop-problems-please

or look at my post when I had this issue:

http://ugx-mods.com/forum/index.php?topic=2730.msg26642#msg26642

You guys rock, I am now able to kill zombies with my son. Any ideas how to fix the occasional 400 fx error?