Funny enough this is one of the few topics described in the 'manual', so RTFM http://domotiga.nl/projects/domotiga/wiki/User_Manual
It describes how to:
Create a trigger for every minute.
Create a condition which is true for a given time before or after sunset/sunrise.
Var_Sunset is calculated every day, so a compare against it (which is what the = does) is True if condition is met.
There is an issue with SunSet and SunRise is not correct when Daylight Saving Time is active, will add a workaround for that soon.
Until then just add or subtract the extra hour difference.