Outlookのシステムメニューへ項目を追加してクリックしてもイベントを受け取れない

VSTOでOutlookのアドインを作成しています。
Outlookのシステムメニューへ項目（"About..."）を追加しましたがそれをクリックしてもイベントを受け取れません。

具体的には、以下のGetWindowLongのリターン値が０になります。
lngWnP = GetWindowLong(hwnd, GWL_WNDPROC)
もし、SetWindowsHookExを使用すれば可能であれば具体的にどのようにすれば良いのか教えてください。

VB.net
1Public Class OutlookMenu
2    ' メッセージ処理関数用デリゲート
3    Private Delegate Function D_MyWndProc(
4        ByVal hwnd As Integer, ByVal msg As Integer,
5        ByVal wParam As Integer, ByVal lParam As Integer) As Integer
6
7    ' ウィンドウをサブクラス化するAPI
8    Private Const WM_SYSCOMMAND = &H112
9    Private Const GWL_WNDPROC = -4
10    Private Const MF_SEPARATOR = &H800&
11    Private Const MF_STRING = &H0&
12    Private Const IDM_CUSTOM As Integer = 1010
13
14    Private Declare Function FindWindow Lib "user32.dll" Alias "FindWindowA" _
15        (ByVal lpClassName As String, ByVal lpWindowName As String) As Integer
16    Private Declare Function GetSystemMenu Lib "user32" _
17        (ByVal hWnd As Integer, ByVal bRevert As Integer) As Integer
18    Private Declare Function AppendMenu Lib "user32" Alias "AppendMenuA" _
19        (ByVal hMenu As Integer, ByVal wFlags As Integer, ByVal wIDNewItem As Integer, ByVal lpNewItem As String) As Integer
20    Private Declare Function GetWindowLong Lib "user32.dll" Alias "GetWindowLongA" _
21        (ByVal hwnd As Integer, ByVal nIndex As Integer) As Integer
22    Private Declare Function SetWindowLong Lib "user32.dll" Alias "SetWindowLongA" _
23        (ByVal hwnd As Integer, ByVal nIndex As Integer, ByVal dwNewInteger As Integer) As Integer
24    Private Declare Function SetWindowLong Lib "user32.dll" Alias "SetWindowLongA" _
25        (ByVal hwnd As Integer, ByVal nIndex As Integer, ByVal dwNewInteger As D_MyWndProc) As Integer
26    Private Declare Function CallWindowProc Lib "user32.dll" Alias "CallWindowProcA" _
27        (ByVal lpPrevWndFunc As Integer, ByVal hwnd As Integer, ByVal msg As Integer,
28         ByVal wParam As Integer, ByVal lParam As Integer) As Integer
29
30    ' デフォルトのメッセージ処理関数
31    Private Shared lngWnP As Integer
32
33    ' 独自メッセージの処理を開始
34    Public Shared Sub Start_MyWndProc(ByVal hwnd As Integer)
35        lngWnP = GetWindowLong(hwnd, GWL_WNDPROC)
36        Call SetWindowLong(hwnd, GWL_WNDPROC, AddressOf MyWndProc)
37    End Sub
38
39    ' 独自メッセージの処理を終了
40    Public Shared Sub End_MyWndProc(ByVal hwnd As Integer)
41        Call SetWindowLong(hwnd, GWL_WNDPROC, lngWnP)
42    End Sub
43
44    ' ウィンドウに来たメッセージを振り分ける関数
45    Private Shared Function MyWndProc(
46        ByVal hwnd As Integer, ByVal msg As Integer,
47        ByVal wParam As Integer, ByVal lParam As Integer) As Integer
48        If msg = WM_SYSCOMMAND Then
49            If wParam = IDM_CUSTOM Then
50                ' 独自メニューの処理
51                MsgBox("VB Web About...", vbInformation, "About")
52                Return 0
53            End If
54        End If
55        ' デフォルトのメッセージ処理
56        Return CallWindowProc(lngWnP, hwnd, msg, wParam, lParam)
57    End Function
58    ' ============================================================================================
59    Public Shared Sub customMenu()
60        Dim hwnd As Integer
61        Dim lhSysMenu As Integer, lRet As Integer
62
63        hwnd = FindWindow("rctrl_renwnd32", vbNullString) 'Outlook本体
64        lhSysMenu = GetSystemMenu(hwnd, 0&)
65        'Add seperator
66        lRet = AppendMenu(lhSysMenu, MF_SEPARATOR, 0&, vbNullString)
67        'Add new menu item
68        lRet = AppendMenu(lhSysMenu, MF_STRING, IDM_CUSTOM, "About...")
69
70        Call Start_MyWndProc(hwnd)
71    End Sub
72End Class
73--