Hello,
I am an administrator of a Discourse forum, but I cannot find how to appoint moderators for specific categories only, rather than for the entire forum.
Thank you very much.
Welcome to Meta 
The category moderator feature is controlled by the enable_category_group_moderation site setting. Is that enabled on your forum?
If so, you should find the option to select the groups whose members should moderate the category in the category settings. You visit the category and click on the wrench.
Then you enable the advanced settings, select the moderation tab, and there you should find the setting to add the groups.
