This guide will outline how to upload and use your own JAR file, whether it be a developer build of Craftbukkit, Spigot or the latest minecraft snapshot.
*Note: You can find a variety of popular modpacks from the mod drop-down menu on your multicraft control panel, which doesn't require you to manually install the JAR File.
1. Download the filezilla client from here.
2. Use the following FTP file details to connect to your server:
Host: You can find this by clicking on the FTP File Access section of your multicraft control panel.
Username: You can find this by clicking on the FTP File Access section of your multicraft control panel.
Password: This is your multicraft password.
3. Click 'Connect' to connect to your server FTP.
4. Create a folder in your server directory and name it 'jar' if one isn't already present.
5. Upload the JAR file of your choosing to the folder. Here are some useful download links for the latest build of Spigot, Craftbukkit and Vanilla:
For Vanilla: https://minecraft.net/download
For Craftbukkit: https://dl.bukkit.org/downloads/craftbukkit/
For Spigot: http://ci.md-5.net/job/Spigot/
6. Go back to your Multicraft control panel and type in the name of the jar you've just uploaded - make sure to type the name precisely.
For example, if you uploaded craftbukkit.jar, type in craftbukkit.jar where it says 'JAR File'
7. Click save and restart your server.