It is quite strange the way to use them.
2. There should be a pull down menu where your paintbrush(es) should show. If they do NOT show then you probably have them set on a trade in the Trading Post. Get them out to be able to continue.
3. Choose a neopet to use it on. Your chosen neopet MUST be able to be painted with that paintbrush.
Meanwhile, you must be careful! The Pant Devil or Grundo Leader  can come and swipe it while you get there. If one of them steals it, say good bye to your 100,000 neopoints (or more) investment as TNT WILL NOT replace it; as this is part of the "features" in the Neopets website.