Login credentials will be sent to your registered email ID after submitting your details. If you do not receive your credentials within a few minutes of submission, please check your Spam folder just in case the email got delivered there instead of your inbox. If so, select the email message and click Not Spam, which will allow future messages to get through.
In case you do not receive an email in any of the folders then write a mail to admissions@iimmumbai.ac.in for any assistance.
CAT Registration Number will be your username.