@logical: You can set the gamemode via console by using the map command. <map "mapname" "gamemode"> This works also for coop modes like "map congress_night conquer" but I recommend you to launch a map from the main menu with the gamemode of your choice before changing the map to congress_night. Especially if you want to change from a PVP gamemode to a cooperative gamemode it requires to initiate the gamemode manually from the menu to prevent AI issues during the game.

how do i choose the game mode?

To add nav file:

~
sv_cheats 1
nav_generate

no nav file ,so all players will die at beginning for this...(any help?)

no nav file ,so all players will die at beginning for this