The main difference between Presonus Studio One Artist and Studio One Pro DAWs the first one allows to use only built-in effects and just EZDrummer, Guitar Rig 3 and Kore Player as external VSTs.
Last holidays I tried to install Studio One Artist (bought for $20 on NAMM special) and I had problems with recognition of EZDrummer Lite and Guitar Rig LE plugins shipped with it. After some investigation with Microsoft Process Monitor I found that Studio One Artist does search of plugins only in default VST directory but I installed them in another place. I changed default VST directory in HKEY_LOCAL_MACHINE\SOFTWARE\VST and the problem was solved. Read more of this post