The node under the Sentinel is out in the open and is one of the six the Xeochicatl pick from at random.
Where it is
Under the Shadow of the Sentinel. The marked map is on the location page: Under the Shadow of the Sentinel.
Getting it
No trick. Use the node where it stands.
Melding
Stand within three squares of the node and use it. The bond raises your maximum mana by three points plus one for every ten points of Mysticism above five, so between three and eight depending on how mystical you are. You can bond with each node once, and the game will list the ones you hold if you ask.
This node can be drained by the Xeochicatl. See Nodes for what happens when one is attacked.