Know the New Hiking How-tos
Hiked this on May 15, 2010, just with this description, no map. The trails are very well blazed, with easy-to-follow cairns and rock lines in the treeless spaces. I could not find the trail to the Megaliths, tho.
The directions seem to read that you will reach the blue dot Western Trail very soon after the deep fissure, but this is not the case. You will pass over a woods road first, into someone's private tree farm for a while before you get to the Western Trail.
If the trailhead parking is full, drive up to the viaduct and see if there is parking available right underneath it.
Also consider driving over to Orrs Kill Road and Jackson Road for a full-length east-facing view of the viaduct. It is the tallest, highest railroad trestle east of the Mississippi.
Photos from this hike are here: http://www.flickr.com/photos/kleepet/tags/schunemunkmountainloophike/