1. Enter the mansion from the front.
2. Click any of the 4 glowing circles.
3. Clear the room.
4. Click the lamp with the ! above it.
5. Enter the room that opened (2nd floor).
6. Walk straight through the room (don't turn).
7. At the end of the hall (by the corpse, also a quest piece, "Missing Husband") enter the room on the left.
8. Click any of the glowing circles.
9. Leave the room and return to the main room of the mansion (the room right after you enter the mansion) and click the glowing circles again.
10. Leave the mansion and head to the basement.
11. In the 3rd room of the basement click any of the glowing circles.
12. Leave the basement and re-enter the mansion.
13. Click the glowing circles in the main room of the mansion.
14. Clear this room again.
15. Enter the room that opened (2nd floor).
16. Head down the hall and then take the only right. At the end of this smaller hallway, take another right and enter the room that's there.
17. Click the glowing circles.
18. Head back to the main room and click the glowing circles there.
19. Finally, head back to the 2nd floor and enter the room (the room unlocked by clearing the mansion right after you enter).
20. Head down the hall and click the lone yellow glowing circle.
21. You've successfully unlocked the attic. Clear this room to gain access to it.