To receive 100NC for free, simply log in to Monkey Quest (you can log in with your Neopets username and password) and then visit the NC Mall.
http://www.monkeyquest.com/en/home
It can take several minutes for it to appear. Monkey Quest is part of the Nickelodeon group so it's 100% safe for those who were wondering!
Also, there's a free Goodie Bag up for grabs!
http://ncmall.neopets.com/mall/shop.phtml?page=&cat=
Holly Tree Goodie Bag