copy the duke3d and duke3d% to PSP/GAME150 on your memory stick on custom firmware.
If you are on firmware 1.5 just copy the 2 folders to PSP/GAME.
After this is done you need to copy the file duke3d.grp of your orginal duke nukem 3d version to the folder duke3d on your memory stick.
not duke3d% !!!

99% credit goes to MrPeanut.
and 1% to us becouse we made the icon, background and song.