From the Annesburg dreamcatcher head southwest through the woods and up the hill. You have to just go to the marker and look around. The tree doesn't stand out from the rest of the trees here and the dreamcatcher is a little harder to spot from a distance because it is up higher in the tree. Remember to use Eagle eye if you have trouble seeing it.
Finding all 20 dreamcatchers will reveal the location of the Ancient Arrowhead treasure; A unique item that makes Arthur's stamina last twice as long when using the bow.