Default it's not possible to login with root on an ESX host with Putty or such a like tool.

You can enable this to change the following (do it at your own risk).
 

• Use the console (Physical at the server) and login.
• cd /etc/ssh
• Use the command vi sshd_config.
• Change the following at the line where stays PermitRootLogin no to PermitRootLogin yes
• Save and quit VI (:wq)
• Restart de sshd service with the following command service sshd restart.

NOTE: I recommended to use a non-root account for remote ssh connection, and then use the su command to switch to the root account if neccesary.

Download: Putty