Using Application Events With An Add-In

I've created an addin to reformat spreadsheets that I receive in a particular format. What I would like to do is enable events so that whenever a spreadsheet is opened the reformatting procedure is run (this also validates whether the spreadsheet is of the correct format).

I have created a class module with the following code (exactly as the Excel help):

Public WithEvents App As Application

Dim X As New EventClassModule

Sub InitializeApp()
Set X.App = Application
End Sub

Private Sub App_WorkbookOpen(ByVal Wb As Workbook)
SortE1Output 'This is my procedure that determines whether the
'spreadsheet is of the correct format and then reformats it
End Sub

"After you run the InitializeApp procedure, the App object in the class module points to the Microsoft Excel Application object, and the event procedures in the class module will run when the events occur."

When I try to make a call to the InitializeApp procedure in the auto_open procedure (in a different non-class module) I get a "compile error: sub or function not defined".