Bring down the console and type "sv_cheats 1" (minus the quotes) and press ENTER. You can also type
map to load a specific map and have the cheats enabled once the map loads. sv_cheats 1 is write protected in the regular single player game. You need to load a map first using the map or devmap command. So you would type map pra1 or devmap pra1 at the console and hit enter. When the map loads, use sv_cheats 1 and hit enter to enable the cheats.