Event Sink store-wide events

I created an Exchange event sink library to catch the events when appointments are changed.
Now I am trying to register for store-wide events
http://msdn2.microsoft.com/en-us/library/ms878583.aspx

But when I tried doing this
>cscript RegEvent.vbs Add "OnSyncSave;OnSyncDelete" MyEventSink.SyncEvents file://./BackOfficeStorage/mydomain.com/MBX/SystemMailbox{E87248FF-67DA-4848-9557-F0123EC47CE4}/StoreEvents/GlobalEvents/testSyncGlobal

I got this error
Error Opening Record : -2147024891 Access is denied.

I was alredy logged in as Administrator. What did I miss?
Thanks in advanceStart Free Trial