Zillow Rental Manager will use your existing Zillow account. You can access Zillow Rental Manager or create a new account by going to www.zillow.com/rental-manager. Click on "List your properties for free" and either choose to log in to your account, or create a new one and you will be directed to your Zillow Rental Manager account.
After that, you should be all set to start with Zillow Rental Manager!