In the Browser window, please go to "Effects" and choose to sort by vendor. The VU Meter should show up in this list under "PreSonus." You may want to click the icon of the wrench in the browser and make sure that there is a white dot next to the VU Meter in the event that you accidentally hid it from the plug-in manager.
If you are still having a problem, try installing it once more from Studio One>Studio One Installation. Choose to install from your account, select the VU Meter, and choose to install all content and not just the missing content.
If you continue to have a problem, download a fresh copy of the VU meter from your My.Presonus.com account.
Open Studio One and go to the Studio One menu Help > Open Settings folder, close Studio One and delete the presonus.vumeter.win folder out of the Extensions folder. Browse to the location where the VU meter was downloaded, double-click it to install the VU Meter once more.