If at anytime you need to disable the server's online-mode setting to authenticate with the Mojang servers, you can do so by following the steps below:
- Login into Multicraft
- On the Left Hand Panel, select "Config Files"
- Go into the Server Settings file.
- Scroll down to Online Mode
- Set Online Mode to Disabled
- Press Save down the bottom of the page, and Restart your server.
Note that by disabling online-mode, you are essentially turning your server into a "cracked server" where users without a legitimate copy of Minecraft can join your server, which does increase the odds that they are using cheats/hacks. This also durastically increases the risk of your server being hacked, by someone logging into with your username.