Sorry to hear you're having issues with this. I've managed to get over the wall and successfully submit MTD VAT returns for a client so I'm hoping I can help you over it too!
Quickbooks recently hosted an MTD day and they had a very comprehensive guide on how to sign up with HMRC, assign clients, switch on MTD in QBO and file, so it may worth a look at their on demand webinar from the day, link below:
If you are initially looking to sign clients up and do not already have a VAT agent account this will be your first port of call. If you are on your old account and already have clients, you will need to 'link' your existing clients with your new agent account and you will then be able to use your new User ID and password in QBO to submit.
Alos be aware that your clients need to be eligible for the pilot if you plan to sign up early and you need to also sign them up and receive confirmation from HMRC before you switch on MTD in QBO as this can't be switched off.