I found this:

This hack is for multiplayer !  

install CWcheat

use CWcheat DBE (DataBaseEditor)

//Rclick - Add game
naam: Wipeout Pulse Promo
Sony Code: UCET-00713

//Rclick - Add Code name
Code Name: Unlock Adhoc and LAN

//Rclick - Add Code
Code: "0x0049B3D8 0x00000000"

turn on cwcheat
Start Wipeout pulse
keep select pressed for two seconds
enable cheats,
done! it works!

Also there is a hack here but the site is kinda strange its a hack to unlock other menu item's
Credits go to :
SketcHacker !!

Yah, i also made it so it unlocks other menu stuff too. some don't work, others do. the ONLY part of the menu i have yet to unlock was in extras i think it was...nothing special in there anyways but...

Im guessing the code of the game wasn't done enough to get online mode working, but i figured hey, if ADHOC modes work, what the heck. as this was a demo, and was not intended for you to play MP, i hacked em open, nuff said lol.

Anyways, i plan to make other things open up, like all tracks and vehicles. i know there is other cars/levels that's in the demo that's only going to be playable in the full game, so when i get around to it ill finish up unlocking everything else.

By the way, anyone can learn how to do stuff like this. my website, wee teach new comers how to create there own codes for psp games. not hard to learn either.
Oh and you can just fire up CheatMaster, enter this code in, DONE. CWCheat is good for USING codes yes, but too much you have to do in order to enter just one code. connect to PC, edit a file, save, reupload to PSP etc etc etc. CM, just open, add, use. its also the tool that i made the UNLOCK MENU code above with also.

http://www.OneHitGamer.com