The plug-in should work after having added the Mastering Suite in your project in the last effect slot of the master bus (2019.2) or the System audio device (2021.1). If that's not the case, profile your game and verify if there are any errors reported in the Capture Log.
Note that if you installed the Mastering Suite after you have integrated Wwise into your project, you need to modify your integration from the Unreal tab of the Wwise Launcher so that the plug-in libraries are correctly copied to your project.