Attempt at using plugins for iphone

Following what I have read on the Unity site I've been attempting to implement an objective c plugin for an iPhone app. However with even the most basic examples I try the Xcode project will not build without errors.

With:

#import <UIKit/UIKit.h>

extern "C" { float TestPluginFunction() { return 5.0F; } }

and

using UnityEngine; using System.Collections; using System.Runtime.InteropServices;

 [DllImport ("__Internal")]
 private static extern float TestPluginFunction();


 void Awake(){
     if (Application.platform != RuntimePlatform.OSXEditor){
         print( TestPluginFunction());
     }   
     else{
         print("blah");
     }
 }

}

I get the error:

Undefined symbols: "_TestPluginFunction", referenced from: _TestPluginFunction$non_lazy_ptr in RegisterMonoModules.o (maybe you meant: _TestPluginFunction$non_lazy_ptr) ld: symbol(s) not found collect2: ld returned 1 exit status

It is probably something obvious - can anyone see what the problem is?