You probably just need to log out and log back in.
But you can also start Joplin manually, please take a look at this thread for instructions on how to do that.
I suspect that you won’t get the integration prompt, but if you do select yes and you should be all set.
Also if you can, please let me know whether or not you get that prompt, it’s possible that lubuntu isn’t properly supported, in which case the script needs to be updated.