Hello,
You will need to purchase the VST/AU plug-in support add-on to utilize 3rd party plug-ins with Studio One 3 Artist. You can purchase the add-on here:
https://shop.presonus.com/products/software/studio-one-add-ons/VST-and-AU-and-Rewire-Support
Once you purchase and install the add-on you will be able to add 3rd party plug-ins to the plug-in directory in Studio One 3 Artist.
If you have further issues please submit a technical support request so that we ay further assist you.