Well, assuming you've actually located the editor in the tools folder, open it up, and pick Open from the File menu to use an existing racial tech tree as a base, or enter an Internal Name and Display Name right now to start one from scratch.
The existing racial tech trees are under Data\English\TechTrees\ by the way.
You must have a new Internal Name for the tech tree to be considered different from the original.
You want to save your racial tech tree to a mod folder under Mods\YourModFolder\Data\TechTrees.