[collectd] how to manage alert

[habib]

basically what i need to know is how to for example make a message pop 
up (std output or whatever...) when there's a ping failure.
I manage to monitor (with rrd) cpu graphs... but i don't really 
understand what i need to use to generate an alert on a problem. Should 
i use the exec plugin and if yes i do't understand how to use it (even 
after carefully reading the man page).
So could you please provide me a lead to do what i want or an example 
that would help me understand the way collectd (and exec plugin) manage 
alerts and whatever come along...

Thank you for your answer. Don't hesitate if you need me to provide 
config files or whatever

Habib Daoud