Hey there here are my two cents.
Hinting the users the flash is interactive: I would say, before the user even touches anything, make the lines seem "floating" on loading. I mean, move them slightly left and right, make them seem "grabbable".
Hinting the users about geometry: How about you simply draw in some faint color ALL the possible options at once, hence creating a very big and complex shape onto which the user will drag the lines and place them in different shapes? I would also suggest that the lines would "snap" to the lines of the faint shape in the background while playing a clear sound.
Give them a choice: How about making a mysterious preloader, that would lure the visitor to go through the puzzle, but will also give him the choice to jump straight to the content if he wanted? Let's say a visitor returns to your site to show a video to a friend, he wouldn't remember the exact combination that led to that specific video. I think that either way, you should put up a "normal" website that would allow for easy access. That doesn't mean that you can't make the option of going through the puzzle seem very lucrative!



