From the page you linked to:
Code in this namespace is not currently included in Noda Time NuGet packages; it is still deemed "experimental". To use these serializers, please download and build the Noda Time source code from the project home page.
For 1.2, we'll be distributing a separate pre-built assembly and NuGet package, but that's not quite ready yet, so for now you'll have to build your own.