When you come to the last stage of Ravenhearst it asks you to collect 7 skeleton keys and go to the cellar. When you click on the MAP you will see that all the tabs are highlighted. Now you will have to go step by step to finish the game.
Step 1: Click on Towers - Find a key in one of the rooms and come back to MAP
Step 2: Click on 3rd Floor and do the same as step 1
Step 3: 2nd Floor - Find 2 keys in any 2 rooms
Step 4: 1st Floor - Find 2 keys in any 2 rooms
Step 5: Ground Floor - Find 1 key in the room
Note: The whole idea is to find 7 keys from Towers to Ground Floor making sure that the basement tab is highlighted. Do not find keys from all the rooms of each floor. Else the basement tab will not be highlighted

Now that you have procured 7 keys click on the highlighted basement tab and go to the Cellar to unlock the door.

I hope this works for you because it did for me. If you are not lucky to get it in the first go then don't worry, let the time run out and do it again. I did it on the 3rd attempt.

Please let me know if it worked for you.

i did as u said, but i still have 3rd floor highlighted... i have tried various combinations and basemend is never available... :(