To enable offline mode simply edit server.properties file.
Go to the control panel > Default config files to edit it.
Set "online-mode" on false, save and restart server.

Attention:
This mode is to be used at your own risk.
- Players with non official (cracked) game will be able to join
- If you do not install an auth plugin such as xAuth or Authme then anyone will be able to join using any name, taking its permissions and ranks

While RoxServers.com tolerates offline mode, we do not provide support for its use or for the consequences of its use.