When entering the museum at night, you slip past the security office and then avoid the lasers in the statue room. When you reach the painting, jump onto it. This may not seem wise, since you immediately get caught...but it is the next step in the quest.
look up the walkthrough on youtube its so easy.
Just run past the window when the guard's shadow isn't visible on the shades.
There are no snowballs on Counterfeit Island. (see the related question)
Go through the hatch and try to get past the museum security till you get to the scream painting.
Wait until her shadow moves away from the Security Room window, then run across.
To pass the Security Room, wait until the guard's shadow is no longer on the window.
You have to get a balloon.
In the upper left of the museum.
the security guard is at Bobo's clown store
you have to take the underground tour
He is in the BoBo Clown Store
in the lighted room on the second floor