And to add: if you go to the mistlands in your old world that you've already explored, it will still have nothing. You'll need to find a fresh mistlands that you haven't gotten too close to for it to spawn at all.

And as an extra note, I've heard that it doesn't spawn properly on old worlds in the first place at this time. If you go there, you'll likely find most of it an a bunch of rocks jutting out the water, nearly untraversable. You'll want to start a fresh world if you want to see the biome.