Normally, plug-ins and other settings are preserved across registration, re-installs, and updates/upgrades, so not sure what's going on here. But you should be able to re-add the plug-ins. Go to File > Plug-in Manager, click Add, and navigate to the folder where you placed the plugins (a plugin appears as a ".lrplugin" file on Mac and a ".lrplugin" folder on Windows).
Thank you much. I'll give it a try and let you know if that does the trick.