Sometimes, the SSL Meter plug-in does not show up in the plug-ins list in Studio One, even after you have installed the Meter plug-in.
In order to resolve this, you may need to use the 'Remove Plug-in Settings' button in Studio One in order to force Studio One to do a full re-scan.
You can find this in the View --> Plug-in Manager and then click the 'Remove Plug-in Settings'.
After you've clicked 'Yes' on the confirmation dialogue box, you'll need to close down Studio One and re-open. When Studio One re-opens it will do a full re-scan and find the Meter plug-in.