There is probably something else that needs to be done, Sadirose. These Nancy Drew games won't let you go on and open a safe (just as an example) until you've found the clue as to the combination, so even if you know what it is from a walkthrough, you can't use it right away.
There may be something that needs to be done yet--I suggest that you read through the walkthrough up to this point, to be sure you've done everything....
