To find your instance please follow this guide: https://www.creeperhost.net/wiki/books/minecraft-java-edition/page/finding-your-instances
On the instance page of the server you want to add a mod to click on
Mods in the instance navigation list.
Once on the
Mods page there are multiple methods to install a mod. Each method has been described below.
Click on the large textbox that says
Find On Curse and search for the mod you want to install