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:

1. Login into Multicraft
2. On the Left Hand Panel, select "Config Files"
3. Go into the Server Settings file. 
4. Scroll down to Online Mode
5. Set Online Mode to Disabled
6. 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.