I have collected my visa through a visa agency, because I did not have the time to go to The Hague twice time-wise, but I will give you what I know:

they collected the visa at the embassy in The Hague (http://www.iranianembassy.nl/):

THE EMBASSY OF THE ISLAMIC REPUBLIC OF IRAN
DUINWEG 20
2585 JX DEN HAAG

Opening hours visa Section:       14-16 hours

 The documents I have send: the number of the LOI, the visa form (2 times, I wrote both by hand but don't know if that is necessary or you can just copy the first one), 3 passport photo's (hair does not have to covered, also not for the ladies)

 Waiting time for the visa was 5 working days (day in 3-8-2011, day the visa could be collected 8-8-2011).

Fee was 60 euro per person.

Hope this helps!