The steps for installing Tekkit (or other mods such as Voltz, FTB, etc.) in Multicraft are as follows:
1. Request your server be switched to an internal jar instead of our panel jars. This can be done either via a billing panel ticket or emailing us at firstname.lastname@example.org
2. Stop the server then delete all files in your root directory. Please make a local backup of anything you want to keep before deleting any files.
3. Download and extract the mod server files on your home machine.
4. Run the install.bat to finish populating files.
5. Upload all the Mod server files via FTP to both the "root" and "jar" directories. If you do not see a "jar" directory you should create it.
6. Rename Tekkit.jar (or other mod.jar) to custom.jar in your "jar" directory.
7. Switch your jar file in the main control panel view to "Mod: Custom" and click save at the bottom of the page.
8. Restart the server and connect using the Technic Launcher.
*Note: Only Sand level packages and above can use custom jars such as Tekkit.