How to import dll from script

Hi, I'm new to unity3d. I want to ask how to import .NET dll to be used in script?

I tried create a hello.cs
using System;

namespace test {
public class hello {
....
...
}
}

compile using mono compiler. gmcs /t:library hello.cs
copy the dll file into projrct folder

in the script,
using test;

but I get an error code.

 

Ok, solved. I have made a silly mistake. There is an old testing script in the project.