You can set the root password (and allow root login) with:
$ sudo passwd root
By default, the root account password is locked in Ubuntu and sudo command is used when root priviledges are needed. IMHO this is a good approach since there really is no sane reason to log in as root on a desktop system.
Here's a good explanation why Ubuntu uses sudo (and has disabled root logins) and a list of benefits and downsides of using sudo: