No special access issues
While on the mountain road from Palaia Penteli to Nea Makri, pass the Karakanta fountain and after 1 km or so and turn on the first dirt road on your right. This is a crossroad of 3-4 dirt roads and there is plenty of room to park there.
If you want to move on for the Manitari Block, by car or on foot, keep on the slightly downhill road on your right and at the first fork take the left direction for another 1 km or so. The block is very obvious and close to the road, at the right side of the hill.