I used to have a Category window on the bottom right panel. But I don't have it any more. So how can I get it back?
See the menu item View/Panels at the top of the IMatch window. This lists all the available panels, including those that have been closed. Just click Categories to get it back, or use the keyboard shortcut F9, C.
The Panels (https://www.photools.com/help/imatch/#panel_basics.htm) topic in the IMatch help explains how to open, close, move, group and dock panels.
I've also made a video titled "Using IMatch Panels and Workspaces" which is available for free in the IMatch Learning Center (https://www.photools.com/imatch-learning-center/)