Error 102 Error registering plugins and/or workflows. Assembly bin\Debug\MyPlugin.dll could not be found.
could possibly be caused for many reasons. For me it has been always one of the following:
1. As I use the CRM Developer Toolkit, and sometimes I add an existing project to the plugin solution, I forget to add the Plugin project as a reference to the CrmPackage project.
2. I also sometimes find that I have used different project framework version from the one used by the package.