Third Party VST integration is not supported with Studio One Artist. In order to be able to utilize additional plugins, you would need to either upgrade to Studio One 3 Professional or upgrade your Artist version by purchasing the VST/ReWire Add-On; which allows for the use of additional non-bundled Third Party VSTs as well as adding ReWire Support in Artist
Here is a Studio One 3 Version chart breakdown of what features are included in each version of Studio One 3:
http://studioone.presonus.com/compare-versions
You can purchase the Upgrade or Add-On here:
Upgrade:
https://shop.presonus.com/products/software/studio-one-prods/studio-one-upgradescrossgrade/Studio-One-3-Artist-to-Studio-One-3-Professional-Upgrade
Add On:
https://shop.presonus.com/products/software/studio-one-prods/studio-one-add-ons/VST-and-AU-and-Rewire-Support