This guide will guide you on how to upload and use your own JAR file, whether it be a developer build of Craftbukkit, Tekkit or any other JAR you'd like to use.
Step 1. Download the Filezilla FTP client which can be found here: https://filezilla-project.org/download.php (You can also use any other FTP client of your choice).
Step 2. Log into your Multicraft -> Click on 'Files' -> Click on 'FTP File Access'
Step 3. Enter the the listed FTP information into a client of your choice. (We recommend Filezilla because it is free and easy to use.)
Step 4. Connect to your server via FTP.
Step 5. Create a folder in your server directory and name it 'jar'.
Step 6. Upload the JAR file you wish to use to that folder.
Step 7. 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. It must be typed exactly as it is named or it will not load properly.) For example, if you uploaded craftbukkit.jar, type in craftbukkit.jar where it says 'JAR File'.
Step 8. Click save and restart your server.