File or assembly name Microsoft.Crm.MetadataService, or one of its dependencies, was not found

Imagine.. You're writing a wonderful MS CRM 3.0 addon. After writing lines and lines of code, the time is there to run and test the addon. Instead of seeing your fantasic, state-of-the-art, rocket-science, cutting-edge addon, it is this message that arises:Parser Error Message: File or assembly name Microsoft.Crm.MetadataService, or one of its dependencies, was not found.Great.Now how to solve