The etimer (Event timer) will post an event when the timer is expiring. Since it posts an event you will need to have a process around it to handle the event.

PROCESS_THREAD(etimer_process, ev, data)
{
  static struct etimer et;
  PROCESS_BEGIN();
  /* set timer to expire after 5 seconds */
  etimer_set(&et, CLOCK_SECOND * 5);
  while(1) {
    PROCESS_WAIT_EVENT(); /* Same thing as PROCESS_YIELD */
    if(etimer_expired(&et)) {
      /* Do the work here and restart timer to get it periodic !!! */
      printf("etimer expired.\n"); 
      etimer_restart(&et);
    }
  }
  PROCESS_END();
}

In the above code the process is defined by PROCESS_THREAD which takes a name, an event variable and an event data variable as the arguments. When an etimer expires the event variable will be set to PROCESS_EVENT_TIMER and the event data variable will be set to point to the specific timer. Using that information the above example could also look like:

PROCESS_WAIT_EVENT_UNTIL(ev == PROCESS_EVENT_TIMER && data == &et);
/* Do the work here and restart timer to get it periodic! */
printf("etimer expired.\n"); 
etimer_restart(&et);

But when waiting for this event in this way – all other events will be ignored so the first approach is more flexible as this will enable handling multiple types of events more easily.