So I've found the JJ2+ in the extras and launched it. From what I read online on jazz2online to start a online co-op game I should run multiplayer in any mode and then enter /coop in chat window... problem is that it doesn't work and I can't seem to find any clue why. Could someone help?

Did you press t to chat? Did you check that the bottom left corner of the menu screen says JJ2+ Beta v5.6? This will also change the look of the menu, grant a 800x600 window option among other things.

I'm thinking what happened is that you opened vanilla JJ2 instead of plus.