You also need to reference the Namespace by right clicking References and selecting the namespace.

I know this post is a little old but I thought this may perhaps help someone coming across this post.

If you want to add a third party dll to a project in unity, the best method is usually to add it to the \Assets\Plugins folder, and it'll be included automatically as a reference in your -csharp.sln file. Adding libraries this way ensures you overcome compile order issues when dealing with UnityScript, and ensures that copies of your projects come with their dependencies.

The "Plugins" folder is where you must put any native plugins, which you want to be accessible by your scripts. They will also be automatically included in your build. In Windows, native plugins exist as .dll files, in Mac OS X, they are .bundle files, and in Linux, they are .so files. Like the Standard Assets folder, any scripts in here are compiled earlier, allowing them to be accessed by other scripts (of any language) that are outside the Plugins folder. - http://wiki.unity3d.com/index.php/Special_Folder_Names_in_your_Assets_Folder