Hi Paul,
Make sure you have the latest SDK with the latest registration tool. As Sachith mentioned, check your project whether it is referring to the correct Assembly(dll).
As you are getting this error in Plugin Registration Tool you can try putting the dll in folder in SDK from where you run the plugin registration tool. (CRM SDK\tools\pluginregistration\bin\Debug). It should ideally have been a part of the SDK.
Thanks,
Prasad