1) Norton 360 is absolutely rubbish. Symantec discontinued Norton 360 about 2 years ago. Just because Norton claims it's dangerous, doesn't mean it is. Get a proper A/V from a reputable vendor.
For fun: https://www.youtube.com/watch?v=lvDzarJOJNQ#t=28
2) There's a verbose guide for install plugins by @BoltBait: http://www.boltbait.com/pdn/InstallingEffects.php
3) Make sure you're not using plugins from the Megalo pack.
4) The Alpha Mask plugin can be found here: http://forums.getpaint.net/index.php?showtopic=1854