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.
Mono.Simd only works when you run your programs on Mono runtime, on a supported operating system. If I'm not mistaken, it's currenly restricted to 32 & 64 bit Linux. Even if you manage to compile it on Windows, you won't get any speed benefit from it.