sv_lan 1 in your cfg/listenserver.cfg. Adding "+sv_lan 1" to your game launch options (when compiling, click advanced) might work, although if cfg/listenserver.cfg sets sv_lan 0 it will probably end up 0.
You could try making a config file e.g. cfg/hammer.cfg, then adding "+servercfgfile hammer.cfg" to your launch options. I'm not sure if that's an option for a listen server, or for GE:S, but if it works it will completely bypass your cfg/listenserver.cfg.