What actually "doesn't work", clearly from your screenshot the plugin runs? Did you try something debuggerish, like setting a breakpoint and running the program to see if it stops?
 Thank you for replying. Here's the running screenshot, compile's fine, build's fine, and execute's fine, but how to start debug??? I've seen someone's instruction that points out the way to use this debug plugin, and that instruction clearly has those debug buttons available, but mine's not, that's the confusing part. (shown below)  As I'm new to Geany, I doubt if there're some settings that I haven't done yet......