Lately Microsoft have changed its layout in Office365 and in that same change made it harder to find Exchange Control (ECP) Panel.
At the moment the easiest way to open ECP is to log in to your office 365 Outlook Web Access.
Then, change the url from something like this: https://pod51049.outlook.com/owa/ to this: https://pod51049.outlook.com/ecp/
As you can see you change OWA to ECP and youre in!
