I went over Lamarck 3 years ago from the east side. In fact that was my first ever backpacking trip! I echo what others have said in that it is not especially difficult to find, but from the west it might be a little harder. If you want GPS coordinates, there are a couple people that have posted their tracks on Everytrail that you can download:
http://www.everytrail.com/view_trip.php?trip_id=296889
http://www.everytrail.com/view_trip.php?trip_id=349998
Keep in mind that these are not mine, and you should use them at your own risk. However, a quick glance at them it seems like they both follow the same route and top out at the correct place. Good luck and have fun! That is a great trip.

