Hi Amy,
The Splunk.Client.Helpers library is only required if you're running the tests that are included with the full SDK package. You'd only need the full package if you're planning on altering or contributing to the SDK.
For most people's purposes, Splunk.Client and Splunk.ModularInputs are all you need. Please see http://dev.staging.splunk.com/view/csharp-sdk-pcl/SP-CAAAEPK for more info.
Thanks,
Matt Tevenan