The sunrise/sunset alarms in your Suunto 5 are adaptive alarms based on your location. Instead of setting a fixed time, you set the alarm for how much in advance you want to be alerted before the actual sunrise or sunset.
The sunrise and sunset times are determined via GPS, so your watch relies on the GPS data from the last time you used GPS.
To set sunset/sunrise alarms:
Scroll to the alarm that you want to set and select by pressing the middle button.
Set minutes in the same way.
A watch face is also availalbe that shows sunrise and sunset times.
Sunrise and sunset times and alarms require a GPS fix. The times are blank until GPS data is available.