FileNotFound when load assembly with dependency to another domain

You may want to tell the domain where to load your assemblies from:

AppDomain domain = AppDomain.CreateDomain("tmpDomain", null, new AppDomainSetup { ApplicationBase = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Plugins") });

However, I don't see why you are loading assemblies in current (default) domain and also the tmpDomain.