Ok, here goes.
Make a folder in your USB Flash Drive called "Music", now....
Put your songs that you want into that specific folder, I don't know if it has to be 10 or more, but I don't think so..
Next put the USB flash drive into the PS3...
Now go to the music section, it may display 2 USB go to the first one and then don't press X but Trianlge, this displays all the files on your USB. (if it doesn't come up then go to the next USB that is displayed.)
Next Go to the the "music" folder that you have created and you will see a list of all the tracks that you have put into it.
Instead of copying the tracks 1 by 1 you should just copy the whole folder. The Ps3 will copy the tracks itself within the folder.
Now that the tracks are on your PS3, you have to create a Folder in the "Playlist" section called KONAMI_PES - make sure that it is exactly like that and that there is no space after. When you type KONAMI_PES and then enter it automatically produces and space just press Square (which is the delete button) and then enter again.
Now that file is created go to it and press edit or triangle to go to edit.
Next you will get a screen that allows you to copy the tracks you have on your PS3 to the KONAMI_PES folder all you have to do is select the tracks and press X and it automatically copies itself.
Now that you have the tracks in the folder, go to the game and then Gallery and then press R1 or L1 to go sideways until you coem to the "MP3 Library" section, now follow what it says on screen like press the tracks and then one of the buttons to add. and there you have it.
If you have any problems then just let me know!!