As contrasutra said there normally is a root account that defaults to no password. If you have had the machine online there is a remote possibility that you got hacked.
The more likely problem is that something went wrong with your install. Sounds like that since you can't find the adduser command.
How did you install, CD or FTP? If CD, did you check md5sum? Did you install all base packages (one of these should contain adduser)?
//Edit: Just thought of something the minute after i posted as usual.
You might try to boot into singel user mode (see this
thread for howto) or boot from CD (provided you have one) and mount the disk. Then check /etc/passwd and /etc/group to see if there is an entry for root. If not you can add one. For /etc/passwd it should look like:
If this doesn't work and you did a cd-install you can try reinstalling (provided the md5sum --check is ok), pay close attention to any error messages on vc/5 (I think thats the one). //