To get the new content, click on the big Download Content button on the main page. There are 2 items for the Halloween special event. The first is the Costume Pack, which gives you these costumes:
- Devil – Monk
- Mummy – Apprentice
- Pirate – Squire
- Witch – Huntress
These are already unlocked and you can edit your character from the character selection screen to pick them immediately.
The second DLC is the Mission Pack, which unlocks the Halloween Spooktacular challenge (in the challenges list). If you complete this challenge on hard or insane, you can unlock the following costumes (depending on the class you beat it with) and a class specific weapon as well:
- Ninja – Monk
- Vampire - Apprentice
- Death Knight - Squire
- Angel - Huntress
I do not think you have to beat the regular missions first, but I am unsure of this as I've alreayd do so.
A recent patch has unlocked the content to Open players as well. So it is now available to all.