To find your instance please follow this guide: https://wiki.creeper.host/books/minecraft-java-edition/page/finding-your-instances
On the instance, you want to add the datapack to, click on the FIles link
Once the files and folders have loaded double click on the world folder (the world folder may vary in name if you have modified the level-name in the settings or imported a world)
Now double-click on the datapacks folder
Once in the datapacks folder drag and drop your datapack zip file into the files list and wait for it to upload
Once its uploaded you will get a notification and you should then be able to use the /reload command in the console of your server or in-game if you are OPped to load the datapack or you can also reboot the instance.