Ken,
I saved a workbook with that name an it worked fine so long as Excel is running and that named workbook is open. Try this and watch the immediate window as the open workbook names are printed.
Sub ScratchMacro()
'A basic Word macro coded by Greg Maxey
Dim oXLApp As Object
Dim oWB As Object
Set oXLApp = GetObject(, "Excel.Application")
For Each oWB In oXLApp.Workbooks
Debug.Print oWB.Name
Next
On Error Resume Next
Set oWB = oXLApp.Workbooks("99 - OSTEEN POSTAL CALC.xlsm")
If Err.Number = 0 Then
oWB.Activate
Else
MsgBox Err.Number & " " & Err.Description
End If
lbl_Exit:
Exit Sub
End Sub