Capture Active Workbook Events Via .xla Add-in

I wrote a small .xla add-in for Excel, which puts a command bar on top, and provides several buttons that do some tasks on the workbook that are frequently needed in our office. At some point, I needed to catch the event of a newly activated sheet, because my add-in will do something upon this event (namely, change the state of one of the buttons on its own command bar)

After some reading and searching, I managed to accomplish the task, however not completely. I first want to copy my code. The code has some tags in it, in order to make it easy to follow the flow and find out where the error lies. Here is the class module in the xla, which handles the events of the application:

' Class name is EventClassModule
Public WithEvents App As Application

Private Sub Class_Initialize()
MsgBox "Event Class initialized" ' Just to follow
End Sub

Private Sub App_WorkbookOpen(ByVal Wb As Workbook)
MsgBox "WB Opened: " & Wb.Name ' Just to follow
End Sub.......................