Enable cheat mode, then activate the "/map redE3M5" code to advance to episode 3, mission 5. Locate the door that does not have a handle in that level. Enter the "/noclip" code and walk through the door to find another door covered with smiley faces.
Cheat Codes
Load the game with the "hl.exe -dev -toconsole" command line. Then while playing the game, press ~ to display the console and type "sv_cheats 1" to enable cheat mode. Then, press ~ and enter one of the following codes to activate the corresponding cheat function: