Hi canucme,
I no longer have my saves for this game, but here's what worked for me:
Stand Sylvie a little way over to the right of the block, you'll see that your cursor will change to show that you're in the right area.
Observe the dark line running across the back inside wall of the hole. As the block starts rising back up towards this line, immediately throw the stone on the crack.
If you still have problems, please let us know. Another member may be able to help with a save.