You need to tell StructureMap how to wire up all types that don't follow the default convention (which you use in your scan). The default convention is to wire up all interfaces to concrete implementations with the same name as the interface (minus the "I" prefix).
In your case you need to tell StructureMap to wire up all needed closed generic types of IMapper
using:
x.For(typeof(IMapper<,>)).Use(typeof(MyConcreteMapper<,>);
Put the line above where you currently have the x.For<IExample>().Use<Example()
.