I had this same problem when installing the exact same model printer in Ubuntu 11.04. I was able to find a solution at that time through Google, and wrote it down. Follow these steps:
Open a terminal window by pressing ctrl-alt-t.
Type: sudo passwd root. This will create a superuser account and establish a password.
Type sudo passwd -u root at the next prompt. This will unlock the superuser account.
When I did this, I chose a different password for the root login, and I used it when running the dell software install shell. It worked. In retrospect, I would probably just use the same password you use whenever you authenticate. The main thing is you have to use the above commands to establish a superuser account.
BTW, I have the exact same printer, and have been unable to print since migrating to Ubuntu 11.10. It now tells me the system can't find the printer. I've run the Dell install several times to no avail. Good luck getting it to work!
(I'm thinking of buying an HP printer since I've heard they're more compatible with Linux).