Need Help with User Category Plugin

[ommaandnugs]

I installed the plugin and set up the users, adding to the user categories by tags (i.e. User Bob includes Action, Adventure, Mystery, etc.). I want to exclude some authors with certain tags from the user category (i.e. under the Tag Mystery is the author Peters, which I want to remove from Bob). I highlight the author, right click, go to user category, which gives a drop down list, go to Remove selected authors, which gives a drop down list of all the user categories--but the user categories are greyed out and I cannot select the one I want. What am I doing wrong? Thanks for the help