Do I actually have to be in the game, hit F1, and then type in the commands or is there a way to do it in the console window?  I've tried prefabs build and prefab build, but it just says "Command 'prefab build' not found".

Fixed... plugin didn't load when added at first.

The only reason it wouldn't work is if the plugin isn't loaded or if the client has a native command for that; try the alternative prefabs command or try changing it.