[SOLVED]
Act 3:
She won't show up in [spoiler]the Arcade[/spoiler], and I think McCoy has to talk to her to be able to progress [spoiler](be able to talk to Sebastian again).[/spoiler]
Thanks!

edit: spoiler tags not working?!

Sometimes you have to revisit the arcade few times if you wasn't lucky to spot her there.

Hint: you don't have to leave Hysteria Hall (location) - just walk into the arcade few times and you will see her for sure.

Thanks, did that a dozen times without success!
I think I had to follow other clues before; now she's there.
Cheers!

I had the same problem and I was nearly giving up. Starting the game from last save the next day fixed it miraculously! :-) Played the game through already. Its fun!