I believe ROM manager works after you have successfully gained root on your device and not before. It may have changed since I no longer have an android phone b

I believe ROM manager works after you have successfully gained root on your device and not before. It may have changed since I no longer have an android phone but when I had my droid the process was install a rooted version of your current OS then you upgrade from there. ROM Manager just made the installing other OS's and extras and keeping cyanogenmod up to date easier.