Using C++ DLL's from Unity's C#?

How do I call functions in a DLL from Unity in C#? I have an example C# program that doesn't run in Unity but they just add a "using libraryName;" at the top of a file and it works.

I've placed the DLL in Assets\Plugins and as far as I can tell Unity knows it's there.

I've seen mention of adding "[DllImport ("libraryName")]" above function defines, but I can't figure out how to call functions in the library and have it actually work.

Here this should answer most of your questions:
http://unity3d.com/support/documentation/Manual/Plugins.html

in C#

   [DllImport ("PluginName")]
   private static extern float FooPluginFunction ();

in the plugin

extern "C" {
  float FooPluginFunction ();
} 

and then you can call it in C#

  print (FooPluginFunction ());

Paul’s answer is correct- but I’ll add that you can only call plugins from unity pro. indie version will not allow it.

@paul …how can u edit the dll in the plugin ?? its the C++ dll…how do we open the dll and write

extern “C” {
float FooPluginFunction ();
}

??