Question: I'd like to use Simd in Mono, but the following code is highlighted as error in MonDevop

using Mono.Simd

I already installed the "Mono 2.8 for Windows", what else I need to install to enable Mono.Simd?

In this link
<http://tirania.org/blog/archive/2008/Nov-03.html>
It talk about the tarbal, but after download it an uncompress it (mono-Mono.Simd.Math-942e804), there are many cs file and no dll file.

Answer: 
Did you add the Mono.Simd.dll to the build? MonoDevelop correctly highlights the error because in the assemblies you're using there isn't any Mono.Simd namespace.
Mono.Simd.dll is included in recent Mono releases.

As for the other answer, Mono.Simd works on any operating system on x86 and amd64, not only on Linux.
MonoDevelop for Windows runs on the .Net runtime, while the Mono.Simd.dll that comes with Mono 2.8 is installed in Mono's GAC.

You will need to Add Reference in your project to the Mono.Simd.dll that came with Mono 2.8.

It should be roughly in:
C:\Program Files (x86)\Mono-2.8\lib\mono\2.0\Mono.Simd.dll.