The Eldergleam Sanctuary is located in the central region of Skyrim, just east of the Throat of the World through the river-valley pass heading east from Whiterun. You will know this area by the defrosted glacier-like terrain and the geysers that are giving the region it's uniqueness; there is also a dragon battle just east of Eldergleam Sanctuary.
Upon entry into the sanctuary you come to a large immovable root system. Use the weapon you received from earlier in the quest to cut the roots, they recoil and allow you to get to the tree. Warning, harming the tree will make mobs appear.