linking problem with box2d

Hi,
I am new to cocos2d-x.
After setting up the environnement, I tried to do some tests.
So I created a new project and opened the win32 (I am using VS 2012)
It built fine, no issues with the Hello World scene.
The problem appeared when I tried testing Box2D.

const b2Vec2 Gravity(0,0);
b2World* world = new b2World(Gravity);

generates the following error :
Error 5 error LNK2019: unresolved external symbol “public: __thiscall b2World::b2World(struct b2Vec2 const &)” (??0b2World@@QAE@ABUb2Vec2@@@Z) referenced in function “public: virtual bool __thiscall GameScene::init(void)” (?init@GameScene@@UAE_NXZ) C:\dev\cocos2dx\projects\ProtectIt\proj.win32\GameScene.obj

Any idea of what might be the cause of the error ?

no one have an idea ?