after u get the diary, u need to talk to the old hag about charms, then u need to find professor Flitwick (he is in one of paintings ; ) ) and ask him about charms, then he tells u that u need to perform illegitimacy kinda stuff to the lill petite teen girl, then u'll be able to see through her thoughts while drinking beers together, the brighter thought shows the keyword to the obfussus charm applied on the diary, that's all I can tell u for now, try to use the walkthrough provided by the developers too.
BUT, the whole game is still full of bugs (0.3.1F) as I've seen till now, including the ginny side quest in chapter 4 and 5, for me, she just doesn't show up ANYMORE after I mail her UNOFFICIALLY ; _ ( (no matter which day it is ; ( )