From the docs:
On the desktop and mobile apps, an alarm can be associated with any to-do.
How?
select a note, then select "switch between note and todo type" then set an alarm (with the watch on desktop toolbar)
let me know if it works for you as i have some problem with the alarm 
Ah, I needed to make the window much wider to see the alarm clock.
Yet another reason for a single panel mode.
And yes, the alarm works: I get a notification showing the markdown text of the complete note. But only if Joplin is actually running.