The current release 2.5.0 is still available and is marked as the recommended release but because NuGet currently doesn't support pre-release packages — though this is changing in the next version of NuGet with support for the <prerelease> element — when you search for XML-RPC.NET in the Visual Studio "Add Library Package Reference" you only see the 18.104.22.168 dev snapshot release.
To install the 2.5.0 release from within Visual Studio it's necessary to use the Package Manager Console (Tools → Library Package Manager → Package Manager Console). Enter this command:
install-package xmlrpcnet -Version 2.5.0
And the package will be in installed in the project specified in the Package Manager Console:
PM> install-package xmlrpcnet -Version 2.5.0 Successfully installed 'xmlrpcnet 2.5.0'. Successfully added 'xmlrpcnet 2.5.0' to TestNugetApplication.