In my opinion this is a minor update needed to the action instructions as the NCM action isn't clear on the variable to be used.
1) Make sure that your action is returning notes. You can check this by clicking on the Alert once it's fired and checking the "History of this alert on this object" resource which will show the Notes are modified and what the output should be.
It doesn't look like the notes are added to "Alert Notes" resource as I would have expected.
2) This is where what the action says and reality diverge: You need to use the ${N=Alerting;M=Notes} variable. Once you add that to your NPM Event Log action from your screenshot above you should get the notes as you would expect.
