Download the current version of NPlot here: nplot-0.9.10.0.zip.
If you want to use NPlot with GTK#, download this tarball nplot-gtk-0.9.9.2.tar or go here.
The zip release contains source code, example usage in c# and C++.NET, and the binary dlls for .NET 1.1, .NET 2.0 and Mono (1.0 and 2.0 profiles). The API reference is under the doc/sdk/ folder and the demos are under demo/.
If for some reason you would like a previous version of NPlot, download it here. And if you want to give a try to the last development code have a look here
See Roadmap for the direction of the current development effort and Documentation, Examples and Tutorials for usage instructions.