Enabling the root account on MacOS X used to be a major mystery to many people in the previous versions of MacOS X, but the Mac help in Jaguar now contains a detailed description of what to do.
There's also an online version in the Apple Knowledgebase (cache).
In short, use Netinfo Manager and select "enable root user" from the "security" menu.
If you know NeXTStep, there's the old way to just open the Netinfo Manager and add an empty "passwd" entry to /users/root. You can then login as root (or su) without password, and use the passwd utility to set a root password.
There's also an online version in the Apple Knowledgebase (cache).
In short, use Netinfo Manager and select "enable root user" from the "security" menu.
If you know NeXTStep, there's the old way to just open the Netinfo Manager and add an empty "passwd" entry to /users/root. You can then login as root (or su) without password, and use the passwd utility to set a root password.