Pokemon news from Down Under.
The unknown dungeon is north of Cerulean City and is the location of the ultimate psychic Pokemon Mewtwo. To be able to enter the unknown dungeon you first have to defeat the Pokemon League.
There may be more quests that need to be fulfilled before the way into the unknown dungeon opens, such as getting the National Dex, but I am not sure.
Try beating the Pokemon League and if that doesn't work then more research is needed!