OK, this is how i softmodded my GC,
I hope this helps you, if it doesn't, sorry
(not mine just copy-paste from document i have):
PHANTASY STAR ONLINE 1&2 + BROADBAND ADAPTER METHOD
===================================================
You will need 2 memory cards for this. One for holding your save
character from PSO, so you can load DOL files. This doesn't need to be
a Nintendo memory card, it can be a 3rd party one. The other one will
need to be an official Nintendo Memory Card, 59 or 251 blocks.
Insert the official Nintendo Memory Card into SLOT B, and insert the
other one into SLOT A.
Step 1: You need set up your PC and Gamecube (with broadband adapter)
to talk to each other. If you have a router, then connect both the PC
and Gamecube to the router, or you can choose to use a cross-over
ethernet cable connecting your PC straight to your Gamecube's BBA.
Then, on your PC, set your internal IP address to 192.168.1.100,
subnet mask as 255.255.255.0. You can do this in the control panel,
network settings for your network card.
Step 2: Fire up your copy of Phantasy Star Online and head on over the
to options menu. Choose network option, provider option, and then
'yes' to go into the network configuration screen. When you are at the
configuration screen select network setup and choose "manualy set an
IP address". Press the next button and enter 192.168.1.32 for the IP
address, 255.255.255.0 for the subnet mask, 192.168.1.100 for the
default gateway, and finally 192.168.1.100 for primary DNS. After
entering in the info press next and save your settings to memory card.
Step 3: Open up a command prompt on your PC: start, run, type in
"cmd.exe" and press enter. Assuming that you extracted all the files
from "GCoS15-install.zip" to "c:\gc", type in "c:", and press enter to
change into the C drive. Then type "cd gc" and press enter to change
into the directory. Next type "psul.exe -s" and press enter. This will
make your PC listen for a connection from your Gamecube.
Step 4: Now on PSO choose the Online Game option at the title screen.
Follow the onscreen directions and create your character. After you
have created your character and saved it to the memory card and the
game asks if you agree to the user terms say yes. The game will begin
to load and you will see a warp hole type screen. At this point the
game is initializing the Gamecube BBA for connection.
Step 5: Back on your computer, you should see that your PC and
Gamecube made a connection. Your PC should have sent data to your
Gamecube, and your character should be saved on your memory card in
SLOT A.
Step 6: Now, in the command prompt, type the following:
psul "1 format.dol"
and press enter. Go back to your Gamecube, and start PSO. Again,
choose the Online Game option at the title screen and select the
character that you created.
Step 7: This time, the game should load a black screen with text. This
step will erase everything on your memory card. Make sure your
Official Nintendo Memory card is inserted into Slot B. Press A to
format it, as the on-screen instruction say. Once it says 'done', turn
off your Gamecube.
Step 8: Take out the Phantasy Star Online disc from the drive, turn on
your Gamecube and go into the GC bios save manager, it should say you
need to format, so format it. Once that's done, turn off your
Gamecube.
Step 9: Go back to your computer and repeat Steps 3-6 for "2 install-
ARcodes.dol", "3 install-level2.dol" and "4 install-gcos15.dol". For
these files, once you start them on your Gamecube through Phantasy
Star Online, press X to write the file, and press R to select slot B.
Step 10: Once you're done with all 4 DOLs, load up Action Replay on
your Gamecube with your Official Nintendo Memory Card in SLOT B. Go to
Start Game, open the lid, close the lid, and GCoS 1.5 should boot up.