I am in the room with all the boxes and barrels. I can't jump across the barrels to the blue box, which I think I'm supposed to do. I have tried to move the boxes, but it doesn't help me, - the barrels are in the way. I haven't had problems jumping before, so I must be doing something wrong.
Can anybody please help me?

You have to push one box (which is standing right at the barrels). Then climb on it. Then push the WOODEN box left of you. Then walk to the other side of the box and push it. Then climb on this wooden box and then to the left (entrance side) and use your mighty whip...
That should do the trick!
GreetZ BiK