He is in the Black Widow's house.
Although you enter the museum to protect the painting, you are arrested for trying to steal it (and so it gets stolen anyway).
go to the far side of counterfeit. on your way, the artist will say something strange about the investigator. continue to her house. use the key you got for the courator. go up to the bedroom and look closely at the painting.
you go back to counterfeit island and find the strange man at the jazz place then u say u framed me! then he runs then u say stop thief and then u get on a scooter and chase until u are at the docks then he gets on a boat.
We never find out, but presumably he is headed to prison with the Black Widow. Unless he got away on the scooter again (the police force on the island is fairly inept).
In Counterfeit Island you go and find the person who stole The Scream that's the painting that gets stolen at one point. Then you must find the criminal who stole it.
He is still on Counterfeit Island because he ends up circling the Island and going to the Inn.
Go back to the museum. You have a package waiting for you there.
You don't. It gets stolen and you have to get it back.
he gets flown into the air with the green balloon you give him
the clown place by the mines
He has gotten away and there is no way to chase him further. Pick up the Key Card he dropped, and go back to the museum. The Assistant Curator will give you a package. There is a hidden message inside that takes you to Early Poptropica island.
Although you enter the museum at night to protect the painting, you are arrested for trying to steal it (and so it gets stolen anyway). When you get past the security room and the lasers, and jump onto the lighting fixture on top of The Scream, YOU WILL BE CAUGHT. This is the way the game is supposed to proceed.