Not sure where you are asking to add the Sunset/Sunrise selection, but I'll try.
If you are selecting sunset or sunrise on the light setup screen I showed above, you just select an option from the drop down list.
If you are using sunset or sunrise in the custom cron window in the cron event wizard then you use @Sunset or @Sunrise there. Like I turn my indoor lights 30 minutes before sunset, so i use (@Sunset - 30).