Apparently this ESC is not a Great Planes brand but is something in the Horizon family as identifed by the orange, brown, and black Rx lead so as a forewarning you will have to reverse EVERY channel if using Futaba, Hitec or Tactic Tx.
Here I installed the spoiler control bracket after making the slot a little deeper and longer. My spoiler only came with what I can only imagine is a reference "scoring" for a notch. Definitely not something you want to install this bracket in. Nothing in the instructions mentions this so I took it upon myself to modify this groove.
Use a hobby knife to deepen and lengthen this groove as seen here. Your model MAY be different as I feel this was an oversight on mine.