Try to re-install the package sudo. If this don't work try enter these in the file manually

#

# This file MUST be edited with the 'visudo' command as root.

#

# Please consider adding local content in /etc/sudoers.d/ instead of

# directly modifying this file.

#

# See the man page for details on how to write a sudoers file.

#

Defaults	env_reset

Defaults	mail_badpass

Defaults	secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"



# Host alias specification



# User alias specification



# Cmnd alias specification



# User privilege specification

root	ALL=(ALL:ALL) ALL



# Allow members of group sudo to execute any command

%sudo	ALL=(ALL:ALL) ALL



# See sudoers(5) for more information on "#include" directives:



#includedir /etc/sudoers.d

If you're already running as root, check to see if the sudoers file exists by typing in:

ls /etc/sudoers

That will kick the file listing out if the file exists. If you have the file available, go with:

# vi /etc/sudoers

You should be able to get to 'insert' mode by pressing 'i' and you will see a notice about it being a read-only file (disregard that since you will be root), enter in the new user, hit escape, and force the file to save by entering in ':wq!'

Hope that helps.