How do I play a LAN co-op campaign in The Lord of the Rings: War in the North?

You can play the campaign with up to two other players.

Playing a co-op campaign in The Lord of the Rings: War in the North is slightly different depending on whether you wish to host the game, or simply join it.

To host a game:

From the Main Menu, click Select Game.
The Select Game screen will appear. If you wish to start a new campaign, select New Game, then select a difficulty level. If you wish to continue a saved campaign, select Load Game, then select the game you wish to load from the list that appears.
You will be returned to the Main Menu. Select Multiplayer.
The Multiplayer Options menu will appear. Select Create Player Match.
Select the character you wish to play as by clicking on one of the models on-screen. If another player has already selected a character, their player name will be displayed next to it and you will be unable to select it.
Once all the players have joined the game and selected their characters, click Select.
A dialog will appear asking whether you wish to launch the game. Select Yes, launch game to begin playing.

NOTE: You do not have to start a new campaign in order to play co-operatively: you can invite other players to take the part of the AI characters in a saved single-player campaign.

To join a game:

From the Main Menu, select Multiplayer.
Select Match Search.
The Games List screen will appear showing a list of available LAN campaigns. Highlight the campaign you wish to join and select Join.
If your game has not been saved at the same chapter and part as the host's, a dialog will appear asking you how you would like to manage saving. Select the option you wish to use. *
Select the character you wish to play as by clicking on one of the models on-screen. If another player has already selected a character, their player name will be displayed next to it and you will be unable to select it.
The game will begin once the host launches it.

* For more information, see Can I play a co-op campaign if I am not at the same point in the campaign as the host? in these FAQs.