For this code to work, you will need a copy of Playstation Magazine Issue 57 (The one with Jango Fett on the cover) and it’s demo disc. From the Half-Life main menu go to option and the the cheats menu. Enter the following cheat: Triangle, Square, Left, Right, Circle, X, Up, Down. It should open an extras menu on the options screen. Go to the extras menu and it should say opening disc tray. When it opens remove the Half-Life disc and put in the issue 57 disc and it should load the Half-Life Uplink Mod. Enjoy!
Bonus Alien Mission
Get an A-grade on all 9 of the Decay missions. If you successfully do this, you’ll get to play a 10th level as a Vortigaunt alien. Getting an A-grade on this level will formally unlock the Alien Mode. (Note: the best way to ensure an A-grade for a level is to get at least 25 kills.)