While I cannot be certain current version is without bugs or even have plugins enabled; this is how I enabled them last time.
Check inside your godot folder. There should be a folder called "demos". Open it and check the examples inside the "plugins" folder. If you just want to test it; copy one (or more) of them inside your project folder and create a new folder called "addons" where you will put them.
If you wanted to try the "customnode", the path would look something like /your-project/addons/customnode/
Then (theoretically) it should appears inside the plugins list, which is found in the upper right corner; Settings -> Editor Settings, inside the "Plugins" tab. If no plugins appears there, try the reload icon near the upper right corner of Editor Settings. If none of that works; well, can't help you until they enable/fix the plugins :(