When you first launch your minecraft server it should automatically create the plugins directory. If not just create a folder named "plugins" in the root directory with your minecraft server JAR file
If you are using bukkit the the plugins folder should be created when you first launch the server. It is located in the same directory as the .jar and the launcher.
Well the creator of the NPC mod said that there was a "Plugins" folder, so yeah...
First get hmod it works with the npc plugin.
viewtopic.php?f=1023&t=147182
If you have problems installing that ask in the thread.
Then install iconomy 1.9 or above. There is a link in this post.
viewtopic.php?f=1023&t=160435
Then install npc's as per this thread.
viewtopic.php?f=1023&t=125389
If you have any questions post in the proper thread.