Yes to clarify,
it works on a new campaign. It will currently not work when plugging the add-on into an already created campaign. Campaigns are handled in a similar fashion to save games and the skill list is not correctly updated if skills get added/removed after creation. We'll fix this asap of course

Sincerely,
Kevin
Did this never get fixed? I came here searching for a solution because I have the same issue. I used the Stat Editor to add one new Skill for my GM Mode mod, I simply found WolfBite and used the "Create Derived Stat" context menu option, and changed the "For Game Master" option to "Yes." The skill does not show up in GM Mode for me to use however.