Here are two important links, that will guide you through all the processes. Login to your account on ACCA website, and you should be able to opt into the degree from there.
http://www.accaglobal.com/uk/en/help/oxford-brookes.html
http://www.accaglobal.com/uk/en/student/acca-qual-student-journey/bsc-degree/eligibility.html
