Please check out this article: http://www.studio-one.expert/studio-one-blog//setting-up-the-universal-apollo-twin-interface-in-presonus-studio-one
However, if you are using any version of Studio One other than Professional:
Third Party VST integration is not supported with Studio One Artist or Studio One Prime. 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
--> IMPORTANT INFORMATION: Here is a Studio One 3 Version chart breakdown of what features are included in each version of Studio One 3. It is VERY important that you read this article so you understand the limitations of EACH version of Studio One 3:
You can purchase the Upgrade or Add-On here:
If you have any further questions about upgrades or VST integration please let me know.