With the new 1.7+ updates, Mojang now requires you to accept their EULA for servers. This is very simple, and just requires editing the 'eula.txt' file in your file manager. You won't be able to join your server until this is changed.
1.) Go into your Multicraft panel, click the "Files" drop down tab and click "FTP File Access". Login to your FTP file manager then edit the 'eula.txt' file. 2.) Change "eula=false" to "eula=true". Then save the changes, and restart your Minecraft server.