Allright, I figured it out. The trick is, that you have to put the crates on the lever, on the side near the wall, where you came from. When you put the crate on the lever, the crate starts to slide (that is wher I went wrong, I thought that the crate wasn't in the right position). After that, you jump from the stairs onto the other side of the lever. Voila, the crates fly..