The default in mint is that you can't sudo. To change this you need to add the acount to the "sudo" group which can be done by Menu -> Administration -> Users and Groups -> Manage Groups -> sudo -> Preferences -> Check user names you want to be able to sudo. This allows the user to sudo using the users password so it is a security (and uninformed user) risk on shared acounts. A different way to do this is to add the following lines to /etc/sudoers
ALL ALL=(ALL) ALL
Which will allow any user to use sudo but will require them to have the password for the user they are sudoing into (default is root).