The "Ping Check" plugin allows you to quickly measure & alert on a servers ping response time, but can also be used to alert when a ping fails. In this case it returns '-1' as result value, which can be used as alarm threshold condition.
To do this, first install and deploy the Ping Check plugin. Once it's up and running, go to Settings -> Alarms.
Create a new Alarm against the ping-time metric. The plugin reports a -1 ms ping time when the host is unreachable, so use that as your threshold. You can also add a message to the bottom that will be displayed when the alarm is raised.
Your Alarm definition should look something like the below:
Once you save the Alarm, it will display on the dashboard like any other by turning your ping-response graph red, you'll also receive notifications if you configured any actions.