ItemTest.mm

//
//  呼び出されるObjective-Cのクラス
//
@interface ItemTest : NSObject
{
    int count;
}
-(int) ResultCount;

@end


@implementation ItemTest

-(int) ResultCount
{
    count += 1;
    return count;
}

@end

Test.cs

using UnityEngine;
using System.Collections;
using ObjC = ObjCMessage;
using System;

///
/// 呼び出すC#のコード
///
public class Test : MonoBehaviour {

	IntPtr instance;
	IntPtr classInfo;

	void Start()
	{
		// instance = [[ItemTest alloc]init]; と同じ
		classInfo = ObjC.GetClass("ItemTest");
		instance = ObjC.MsgSend(ObjC.MsgSend(classInfo, ObjC.Selector ("alloc")),  ObjC.Selector ("init"));
	}

	void OnGUI ()
	{
		if( GUI.Button(new Rect(0, 0, 300, 50), "add") )
		{
			// [instance ResultCount]; と同じ
			var result = ObjC.MsgSend( instance, ObjC.Selector("ResultCount"));
			Debug.Log(result);
		}
	}
}