I contacted PreSonus support, and this is what they suggest:
1. Please open Studio One and go to Studio One>Preferences>Locations>VST Plugins and verify that you have the correct directory added to the list of folders that the program scans at startup. You want to add the directory that you have your plug-ins installed to. If you do not see the "VST Plug-ins" tab under the Locations tab, then you want to make sure that you have activated Studio One Professional or Producer or you have installed the VST, AU, and ReWire support add-on if you are running the "Artist" version of Studio One. Press the "Reset Blacklist" button while you are in this window.
2. Studio One will only load 64-bit plug-ins into the 64-bit version of Studio One. The 32-bit version of Studio One will only load 32-bit plug-ins. If you need to run Studio One in 32-bit mode, simply ctrl-click (right-click) the Studio One icon in your Applications folder and go to "Get Info." You can then check the box to run in 32-bit mode.
3. Please click the icon of the wrench in the browser window, at the top, right-hand corner of the window to open the plug-in manager. Make sure that there is a white dot next to the plug-in, if it shows up in the list.
If you are still having problems, please email firstname.lastname@example.org, referencing this post. We can then get you in touch directly with PreSonus support.