You can get NPlant from nuget. Its distributed in two different packages:
PM> Install-Package NPlant.API
NPlant API: Contains NPlant.dll and NPlant.UI.exe. If you're building NPlant diagrams and running them locally this is the package you probably want.
PM> Install-Package NPlant.NAnt
NPlant NAnt: Contains NPlant.dll, NPlant.UI.exe, and NPlant.NAnt.dll. You'll want this if you want to usese NPlant with your CI process