MFC 入门级问题

照书上写的代码
在红色部分出现错误:error C2065: 'OnLButtonDown' : undeclared identifier
使用的是VC++6.0 选择的是"Use MFC in a Shared DLL"

#include <afxwin.h>
class CHelloApp:public CWinApp
{
public:
    virtual BOOL InitInstance();
};

CHelloApp theApp;

class CMainFrame:public CFrameWnd
{
public:
    CMainFrame()
    {
        Create(NULL,"hello",WS_OVERLAPPEDWINDOW,CRect(0,0,400,300));
    }
protected:
    afx_msg void OnLButtonDown(UINT nFlags,CPoint point);
    DECLARE_MESSAGE_MAP()
};

BEGIN_MESAGE_MAP(CMainFrame,CFrameWnd)
    ON_WM_LBUTTONDOWN()                  //ERROR
END_MESSAGE_MAP()

void CMainFrame::OnLButtonDown(UINT nFlags,CPoint point)
{
    MessageBox("Hello","问候",0);
    CFrameWnd::OnLButtonDown(nFlags,point);
}

BOOL CHelloApp::InitInstance()
{
    m_pMainWnd=new CMainFrame();
    m_pMainWnd->ShowWindow(m_nCmdShow);
    m_pMainWnd->UpdateWindow();
    return TRUE;
}