Remember, the plug-ins are not created by Ambrosia, so we cannot guarantee their performance.
First read the Read Me file that came with the plug-in. Some of them have special instructions to get the plug-in started. If there are no special instructions, then just place the plug-in itself into the Nova Plug-ins folder, and launch the game. If you still have a problem, then the Read Me file should also have a way to contact the author of the plug-in.