User Defined Functions as AddIn

Good morning to everyone.
I posted a question yesterday at
http://www.excelforum.com/showthread.php?t=601728
and the response worked perfectly.

My problem is that I would like to turn my UDF workbook into an addin.

I have placed the UDF code into a module, E.g.

Function Celsius(Deg_F As String)
          ' Convert degrees F to degrees C
Dim myString
    myString = Deg_F
    If myString = "" Then
        GoTo fn_Exit
    Else
        Celsius = 0.0000000001 + (FormatNumber(Deg_F, 6) - 32) * 5 / 9
    End If
fn_Exit:
End Function

In the regular workbook I can place the code for adding the functions to the Insert Function dialog into the This Workbook Open Event

Private Sub Workbook_Open()
On Error GoTo 0
Application.MacroOptions macro:="Celsius", _
    Description:="Converts Degrees Farenheit to Degrees Celsius", Category:="Conversions"
End Sub

Or I can place the code into a module as a subroutine (addFunctions) and call it from the Workbook Open event

Private Sub Workbook_Open()
On Error GoTo 0
    addFunctions
End Sub

All works as it should.
When I save as an addin, I can install the addin and all works.

But as soon as I close out of Excel and open a new Excel window, I get the error message

Run-time error "1004"
Cannot edit  a macro on a hidden workbook.  Unhide the workbook using the Unhide command.

I have deleted the addin, changed the code in the workbook (as above), saved again as an addin, etc.

Everything still results in the same error when I completely close out of Excel and open a new instance.

The code that is high lighted when I click to debug is

Application.MacroOptions macro:="Celsius", _
    Description:="Converts Degrees Farenheit to Degrees Celsius", _
    Category:="Conversions"

What is the proper way of having these functions installed?