Make sure you're logged in to your Minehut panel account. If you don't have one, check out this tutorial on how to create one. Select one of your servers.
You'll want to take it out of hibernation if it isn't already online. Once you've done that, click Edit Server.
Next, click on the Plugins tab.
From here, you can browse the plugins Minehut has to offer.
Simply click on a plugin, and click install.
Keep in mind Minehut only allows you to install a maximum of 12 plugins on the Free plan.