The good news is you've the hard part done, getting the right OIDs can sometimes be tricky.
With the UnDP setup the just open the Alert Manager, open the Trigger Actions tab and at the very top is a drop down menu "Type of Property to Monitor", change this to "Custom Node Poller".
Add a simple condition, for example - "NodeStatus is equal to Down". In this case you would likely want your trigger conditions to look something like
"Poller Name is equal to <poller_name>"
"Status has changed"
Then the rest is just setting up your actions like any other alert.
Hope it helps,
All the best,