Not 100% sure if this is a LinqPad thing, but I'm on the latest beta build (5.07.04) and tried adding a nuget reference to "Birdhouse" and then this script:
var token = "test";
var client = new Birdhouse.NestClient(token);
var structures = await client.GetStructuresAsync();
I'm getting the error "CS7069 Reference to type 'Object' claims it is defined in 'System.Runtime', but it could not be found".
I created a visual studio 2015 console app to do the same thing and it worked fine (albeit in slow-mo compared to linqpad!). Not sure if it's a .net framework version difference??
I've uploaded the script here: http://share.linqpad.net/8og4qn.linq
(Note: if it works it will still error because the token isn't valid, but it will be a more sensible error!)