Nevermind, I'm surprised that such a simple mistake has given me so much trouble haha. I didn't know that I could pass 0 as the joint group parameter jor dJointCreate to allocate the joint "normally" and I thought I had to pass a joint group, so I used the only one I have, the contact joint group, which is emptied each frame. That's why it did nothing.
But now I have another problem: I want the box to slide in a plane, and it does so correctly if the plane has no friction, but as soon as I put a bit of friction into it, the movement starts being a bit jerky, like it moves bit by bit at first and suddenly it gains speed and starts moving smoothly and very fast. Do you know how I can keep the smooth movement while constraining rotation?
P.S: Sorry if I make any grammar mistake, English is not my native language