Strange, i can't find it either. Wtf.
Quote from 69th Vlatitude:
Unfortunately, Valve has not provided us with a complete strings.lst file, so we'll only be able to rely of examples gives from Gearbox's Opposing Force and Counter-Strike
Well, there's your problem.
You can make your own strings.lst file. Simply create a new txt file and put in the following text (As you can see, this is the stings.lst file from Opposing Force):
// String patches
// #define IDS_MAIN_PREVIEWSHELP 400
// #define IDS_MAIN_CUSTOMHELP 530
// #define IDS_MAIN_RETURNHELP 188
// #define IDS_MAIN_NEWGAMEHELP 189
// #define IDS_MAIN_TRAININGHELP 190
// #define IDS_MAIN_LOADHELP 191
// #define IDS_MAIN_LOADSAVEHELP 192
// #define IDS_MAIN_CONFIGUREHELP 193
// #define IDS_MAIN_READMEHELP 194
// #define IDS_MAIN_ORDERHELP 195
// #define IDS_MAIN_QUITHELP 196
// #define IDS_MAIN_QUICKHELP 197
// #define IDS_MAIN_MULTIPLAYERHELP 198
// #define IDS_MAIN_QUITPROMPTINGAME 235
// #define IDS_MAIN_QUITPROMPT 236
402 "www.gearboxsoftware.com/projects.html"
400 "Find out more about Gearbox Software's product line-up."
530 "Select a custom game, maggot."
188 "Back to the current mission."
189 "Begin a new mission, soldier."
190 "Off for a little basic training."
191 "Load a saved mission."
192 "Load/Save a mission in progress."
193 "You looking to tweak out your setup, dirtbag?"
194 "Brief yourself on the current situation, son."
196 "A good soldier never leaves his squad behind."
198 "Prepare for combat. This is not an exercise!"
235 "You going AWOL on me, son? This will abandon the current misson!"
236 "You going AWOL on me, son?"
234 "Leaving for Boot Camp will end your current mission."
338 "Could not validate Opposing Force"
340 "Please Insert the Opposing Force CD"
Simply change the lines to your likings, and save the file. Then change the file extension to .lst. Put it in the right directory inside your mod directory and it should work.