Or is the achievement for it from last year?
All the achievements are obtainable this year. Entrance is in the halloween gamemode, which can be accessed via the second floor of the tower lobby (not the condo lobby).
Wait, even the “Collect 25 pieces of Candy” one?
for that achievement you have to buy a paranormal detector from the Spooky Scary Halloween store and use it to find and collect the Ghost Candy in the plaza which spawns every 6 minutes I think