1. Go to Multicraft and stop your server.
2. Download the modpack of your choice to your computer, and extract the modpack files to a folder on your computer. (Make sure you are downloading the server version of the modpack)
3. If you see any files such as FTBinstall.bat or ServerSetup.jar (or FTBInstall.sh if you have a Mac or a Linux operating system), run this file and it will download all required files. 
4. Once the download is completed, delete all .bat and .sh files. 
5. You should see two jar files within the modpack files. For example: forge-1.7.10-10.13.4.1558-1.7.10-universal.jar and minecraft_server.1.7.10.jar file. Rename the Forge jar file to custom.jar
6. Now using an FTP client such as FileZilla, upload all files (including libraries, mods, config, scripts and jar files) to your server. You can find a tutorial on FileZilla here.
7. On the Multicraft control panel JAR menu, select the "Special: custom.jar" option. This will tell the server to load the custom.jar file.
8. Start your server and it should start up with the modpack.
If you are using a modpack that is older than 1.7.10 Minecraft (For example: 1.5, 1.6.4), instead of remaining the Forge jar file to custom.jar, you have to rename it to customjava7.jar. On the control panel JAR menu, select Special: customjava7.jar.