You want to pick a folder, not a file.
Also might want to set the default initial folder location.
Instead of the MsgBox, set the value of a variable.
Code:
Sub Main()
Dim fd As FileDialog
Dim strPath As String
Set fd = Application.FileDialog(msoFileDialogFolderPicker)
Dim vrtSelectedItem As Variant
With fd
fd.InitialFileName = "C:"
If .Show = -1 Then
For Each vrtSelectedItem In .SelectedItems
strPath = vrtSelectedItem
Next vrtSelectedItem
Else
End If
End With
Set fd = Nothing
DoCmd.OutputTo acOutputReport, "rptEmailREport", acFormatPDF, strPath & "\" & Me.LabelNumber & ".pdf", False, "", 0
End Sub