If MyTools.dll is located in the same directory as Addon.dll, all you need to do is call Assembly.LoadFrom instead of Assembly.LoadFile to make your code work. Otherwise, handling the AppDomain.AssemblyResolve event is the way to go.
If MyTools.dll is located in the same directory as Addon.dll, all you need to do is call Assembly.LoadFrom instead of Assembly.LoadFile to make your code work. Otherwise, handling the AppDomain.AssemblyResolve event is the way to go.