cannot use system.Data in visual studio

I coied system.Data.dll to the assets folder (also I18n.dll.I18west.dll but I still get error trying to use system.data betting error and worning
Warning 6 The primary reference “System.Data” could not be resolved because it has a higher version “2.0.5.0” than exists in the current target framework. The version found in the current target framework is “2.0.0.0”. Assembly-CSharp-vs
how can I fix this? I tried add reference but looking in the properties its stats that it is using the Framework .net version.
on my installation the unity has folders
D:\Program Files (x86)\Unity\Editor\Data\Mono\lib\mono and that folder has 3 folders
2.0 unity unity-web from which folder should I use to copy?
I read that I should’nt use mono but there is no franework in the unity root
I’m using win 7 x64
any help?

other post on this issue stat that th system.data.dll is in

  • List item

/Library/Frameworks/Mono-Framework

In my installation I dont have this folder?