I am trying to follow a tutorial for setting hand positions, but I am missing a button that takes you to the Hand Pose Editor. Any idea what I am missing here?

If you are referring to the bone hand, do the following:-
1.  Bring the character to the Composer Mode. 
2.  Click Content Manager>Body>Hands>G3>Bone Hand>G3 360 Human
3.  Double click Martha_palm front (eg) to replace the existing one.
4.  Use this character as a template to make your own character with the Bone Hand.  
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YouTube Tutorials Here