You need to have registered setup as an option in the Salesforce campaign in order for it to accept the status as registered.
To this, go to the campaign screen in Salesforce and click "Advanced Options". Salesforce will then show a list of the possible statuses, you need to click add and create a status call Registered.
Once this has been done, you can go back to Marketo, create a campaign for members of the program and use the flow "Change status in SFDC campaign".
Let me know if you have any issues.