If the gate on Parnassia is open, get in and locate one of the supervisors walking around. They will allow you to take a few pictures, I did this last friday.
Makes you wonder why an event with such a large regional impact (last time there were 11,000 visitors) has almost no information on the Internet or Facebook.
Well I was there today, and I have seen the fences all around the outside area of the museum. There is a tent for ticket control and access to the area.
Sneaking in is always possible but not advisable i guess.
Probeer de Oude Broekweg vanuit het dorp Valkenburg te bereiken, alhoewel er wegwerkzaamheden zijn deze week.
Als het aan die kant van het veld niet gaat, probeer dan de Ruigelaan (aan de noordkant van het veld) in Wassenaar.