How can I put the multiplayer mode?

[Irilias]

I'm triying to put a player 2, but the game doesn't leave me, what should I do to put it?

[Cyker]

First, you need at least 2 controllers, even if you use only the keyboard for the Player 1, you still need to have 2 controllers to be able to config. So, having 2 controllers, go to options and control config. In this menu you can change what controller each player will use and then you can config the keys.
Now that you have the controllers configured for each player (you can play with Player 1 on keyboard and Player 2 on controller, but like I said, you need to have at least 2 controllers connected in you PC) you will be able to play. BUT, before that, just remember, go to the Ars menu and put the second character to be Manual or Semi-Auto (Auto is for the AI to control).