As a general rule Npgsql can be installed as a simple NuGet from nuget.org/packages/Npgsql. There's a Visual Studio add-in for some Entity Framework operations as well.
Also, you can use NuGet if you want the latest version of Npgsql.
You don't need to install nuget.exe; you can down download the latest NuGet package from http://packages.nuget.org/api/v1/package/Npgsql/
Rename the.NUPKG file as.ZIP and unzip it, and you'll find Npgsql.dll in /lib/net45.
Office.com instructions indicate you'll also need Mono.Security.dll, which you can download at http://packages.nuget.org/api/v1/package/Mono.Security/