It's just extra footage: deleted scenes that didn't make it into the final cut of the movie, plus some extra footage from the cursed videotape. Don't worry, you won't die if you watch it.
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You have to wear 3D glasses (they have red & blue plastic lenses). They usually come with the movie.
Most 3D dvds now usually have the 3d version on one side, & the regular version on the...
I guess in some movie theatres they might do special showings of it, but for the most part, the best plan of action is to either borrow it from the library (or from a friend!), or simply buy the DVD!