Are you wanting to change the MOTD that is displayed to your players in the server list screen of your Minecraft client?
You will need to login to Multicraft and then stop your server.
1. Now on the left side of Multicraft click "Files"
2. Click "Config Files"
3. Then click the "Server Settings" option
4. Scroll down till you see the "Message of the Day" or "Server Message"
The default message you will see in this text area is "A Minecraft Server".
You can now remove this text and enter your own text that you would like displayed in your MOTD.
If you would like to add color to these messages check the color codes for Minecraft here (The color code will need to be placed before the letter or word you want to color): http://minecraft.gamepedia.com/Formatting_codes
5. Once you have done customizing your MOTD you can go ahead and click "Save" at the bottom of the page.
6. You can go back to the home page of Multicraft and then start your server and your MOTD will be live to all of your players!