We'll describe the various expectations of plugins, what goes wrong when they don't meet expectations, and point to general debugging tools that have helped us write crazy useful plugins ourselves.
The node server looks for plugins in its own node_modules directory. A plugin FooBar would be in a module named wiki-plugin-foobar with client/foobar.js within it. Does the server have your module installed alongside dozens more?
The server should know of your plugin by name. Does http://localhost:3000/system/plugins.json show your plugin's name? Try restarting the server and check again.
The wiki client will load plugins and related files by http from its origin server. Watch for this request using browser developer tools. Does it 200? Can you fetch it with a browser at http://localhost:3000/plugins/foobar/foobar.js?