So you've created added a brand new Project to your Dynamics CRM Plugin Solution. You write your code, compile it, and use the plugin registration tool to register the assembly and create your desired steps. You launch CRM to test your new development. You perform an action on the entity in question that will trigger your plugin, and click save. But when the form loads, you receive a generic error.
When you look at the url of the popup, you see it has an error code "8004416c". You know this corresponds with the Error "PluginAssemblyMustHavePublicKeyToken".
You can see in the plugin registration tool that when your assembly is highlighted, the PublicKeyToken field is null. To solve this, you need to have your assembly signed. What you need to do is right click on your project, select properties, and select the Signing tab at the very bottom. Check the box entitled "Sign the assembly", select the file, and save.
Compile, deploy, and register the plugin assembly. Your plugin will now execute without this error.