Just to confirm, you do need to register for Optus Sports separately from My Account - you can check eligibility and register here.
That wasn't the issue Holly. The issue was for people who had already subscribed, but then couldn't login to the service, then having to subscribe again. I've had a tweet from Optus saying that this was indeed a 'workaround'. So there are a few bugs in the process still.
We are aware of some customers having problems with logging in due to information in our systems not marrying up or us needing to push the registration through manually. If you are needing a hand with this please let me know and I'd be more than happy to assist you further.