The Studio One versions that support 3rd party VST/VST3/AU plug-ins are the Pro and Artist (with the VST/AU/Rewire support integration). Please, make sure you have installed the correct version.
Visit the official Studio One webpage for more info!
To load a newly installed ERA Bundle plug-in within Presonus Studio One, you can follow these steps:
1) Create a new session, or open a saved song or project in Studio One.
2) Go to the View menu and select the Plug-in Manager.
3) Inside the Plug-in Manager window you have to make sure that Studio One has properly identified the plugins.
By default, accusonus products will be installed to the system's VST plug-in folder, which is:
For Windows (64-bit), C:\Program Files\Steinberg\VstPlugins\Accusonus
For macOS, /Library/Audio/Plug-Ins/VST
4) In your effects tab on the right of the screen, you should be able to see and load the effects, in the same way as the plugins included with Studio One.
By default, Studio one will place all the plugins by accusonus under a folder titled: accusonus.
To do so, simply drag and drop the plugin on the desired track!