How you add an existing DLL to your project?

I have a dll but i cannot use it in my scripts using "using" and the name of the dll, how can i use their classes??

 

In Visual Studio, you need to Add Reference in your project to the library.
Menu Bar->Project->Add Reference

 

Gives an error saying there is already a reference with that name

 

Ok, i had to use the name of the member of the dll and not the dll name