Penumbra Ouverture strange behavior

[petrosth]

i am a mac user of Penumbra collection games. Just started with the Penumbra Ouverture one. At the end of the first episode, in the workshop, the game does not give me the string necessary to end the level (from the shelves at the left after entering in the wshp). I checked walkthroughs, etc, the game IS supposed to give the string when i touch the box with the cursor. Yet it doesn't, the cursor remains an eye no matter what i am doing. The game is not stuck exactly: I did all the rest, i found the gun powder, placed the TNT, killed the dog, collected all items, etc etc. The game simply denies to me the string, which is absolutely necessary to end the level. I am stuck and can follow no other alterrnatives. Any help warmly accepted