A user cannot telnet/ssh into a BSD box as the user root. For security reasons, it is necessary to log in as another user first and then type SU and enter root's password. SU stands for Super User and basically means you are then logged in as root for the telnet/ssh session, with the appropriate additional privileges.
For this to happen successfuly, the username for the initial telnet/ssh session must be a member of the wheel group in order to SU afterwards. By default, most users aren't members of the privileged wheel group.
To fix this, log onto the console directly as root and type vi /etc/group
Edit the line wheel:*:0:root by adding a comma to the end and then the username to add to the wheel group. For example I modified my group file to read:
Save and exit from vi and then telnet/ssh in as the user who is now a member of wheel. After a successful login you should now be able to SU with root's password. Typing whoami should confirm this shift in user identity.