It was possible to use Bukkit plugins on Forge servers by using Cauldron. However, development for Cauldron has been discontinued since Minecraft 1.7.10 and it is not possible to install Bukkit plugins on Forge 1.8 or newer versions. Luckily, the project SpongePowered was released and you can install Sponge plugins on Forge 1.8 or newer versions.
Downloading Sponge Plugins
Bukkit plugins are not compatible with Sponge. However, many Bukkit plugins have been converted to Sponge. Sponge plugins can be downloaded from here: https://forums.spongepowered.org/c/plugins/plugin-releases
If you are unable to find the plugin you are looking for, you can request plugins on this page: https://forums.spongepowered.org/c/plugins/plugin-requests
Installing Sponge Plugins
Essentially you just need to upload them into the mods folder on your server (not the plugins folder as you would with Craftbukkit/Spigot plugins). This can be accomplished using web FTP in the Multicraft panel (tutorial below) or using an FTP client. If you are not sure on how to setup an FTP client please take a look here.
1. First, download the Sponge plugin from the plugin page linked above. The plugin should be a .jar file. If the file you downloaded is a .zip file, you have to extract it and it should contain a .jar file.
2. Then login to your Multicraft control panel.
3. Now click on Files and then File Manager on the left-hand menu.
4. Now click on the mods folder. This will take you to the mods folder. Unlike Bukkit plugins you have to upload Sponge plugins to your mods folder.
6. Click Upload on the top menu.
7. Now you can drag and drop the sponge plugins to the upload area on the Upload Panel (or click on the upload area to select the plugin files)
8. Once the files have been uploaded, you will see a confirmation message similar to:
9. If you go back to your mods folder, you should see Sponge plugins you uploaded.
10. Now to load the plugins, you have to start the server if it was stopped or restart the server if it is running.
If any information you see here is outdated, please let us know by creating a support ticket. Thank you.