Ok, here's what I just did. I'm listing all of the intermediate steps; if you see something wrong, let me know:
Remove the Pandora's battery from my PSP (just in case).
Plug in AC.
Turn on PSP.
Insert a 1GB memory stick.
Format the memory stick from the XMB (System Settings/Format Memory Stick).
Download Fusa build 47 from this website, and extract the contents.
Enable USB mode on PSP.
Copy the seplugins folder from the Fusa package to the memory stick, overwriting the existing folder.
Check the contents of the seplugins folder on the memory stick:
----The folder contains three files: "FuSa_SD.prx," "game.txt," and "pops.txt."
----The files "game.txt" and "pops.txt" both contain the line "ms0:/seplugins/FuSa_SD.prx"
----There are no other lines in these text files, and there are no other files in the seplugins folder
Copy Gran Tourismo to PSP/Game/
Unmount the memory stick (the right way) from the OS.
Restart PSP, and enter the Recovery menu.
Set Fusa "GAME" and "POPS" to "Enabled."
Exit recovery menu.
Start PSX game. Wait for it to load.
Press and hold -/+ volume keys simultaneously.
Anyone see where I might've gone wrong?
Added (2008-08-20, 9:28 Am)
The solution for me was:
1.Remove all the seplugins you got
2. Also remove conf.bin inside of the seplugin folder
3. Download Fusa, and install it by writing the standard lines to game.txt and pops.txt (make sure you put Fusa first; on top)
4. That's how it worked for me
As noted above, I've met all of these conditions as a result of reformatting the memory stick and starting from scratch. In other words, I don't have any other seplugins, there was no conf.bin inside the seplugins folder (this file is created when FuSa is run for the first time), and the standard lines are already topmost (they're the only lines).
But the comment "make sure you put Fusa first; on top" caught my attention. It seems to suggest that you might have other plugins installed, whereas FuSa is my only plugin. If this is true, what other plugins do you have? I can try installing them, and see if that makes a difference.
That was it. I installed POPS Loader, and now it works. I don't know if FuSa specifically requires "POPS Loader" or if any other plugin will suffice, but I installed POPS Loader, and now it works. Ironically, POPS Loader doesn't seem to work. Maybe it's a bug in the way the CFW handles plugins, maybe it's a problem specific to FuSa, or some combination thereof. For example, if DaX counted from 1 instad of 0 when designing the plugin loader, then all of the lines in pops.txt would be offset by 1 (just a random guess).
EDIT: Yes, this seems to be the case. I added a third line to pops.txt under the first two: "ms0:/seplugins/bla.prx" Then I enabled "bla" in the recovery menu, and POPS Loader works now, too. It's a bug! The last plugin listed in pops.txt won't load. (this is with firmware 4.01 M33-2)