I can’t remember when the change was made to actually installing extensions…and it’s a long time since I’ve used V8. It could be that you were still able to simply drop .rb files into the program’s Plugins folder. Try copy/pasting a single .rb file and see if the extension shows up in one of the menus.
Edit: I just checked and saw that V8 was when the new systm kicked-off. If you really don’t have an “Install Extensions” button on the Extensions tab in Preferences, then you should try a re-install (preferably as Administrator.) If you have the Sketchucation Plugin Store extension, then you can auto-install stuff from there without going anywhere near Preferences.