I had assumed, perhaps wrongly, that the 'install.sh' was getting its buildhost environment information (2.6.38-13-powerpc) from 'uname', and setting the path it looks for the Makefile accordingly. Not having the 'install.sh' script to hand I can't look at it to see why it might fail. Your not really helping in this regard, as your reporting leaves me mostly guessing.
Do you really "get the same error[s]" .. I mean exactly the same error? So, on '3.0.0-15-powerpc' it states:
Or does it in fact read:
Makefile:556: /usr/src/linux-headers-2.6.38-13-powerpc/arch/ppc/Makefile: No such file or directory
If the former then the 'install.sh' has the path hardcoded, and will need to be edited to reflect your particular setup. If the latter then it's looking in the right place but the directory structure doesn't reflect what it expects.
Makefile:556: /usr/src/linux-headers-3.0.0-15-powerpc/arch/ppc/Makefile: No such file or directory
So, if you want me to help, you need to give me more information than you are currently.
As for "Arch/PPC" .. Linux is a case sensitive OS, you do not have "Arch/Powerpc" but "arch/powerpc". Had I not known this, I might have suggested you do something like "ln -s /path/Arch/Powerpc /path/Arch/PPC" which simply wouldn't have worked as these filenames are not uppercase.
I'm partially guessing, but it may be the script is looking for 'ppc' when the Makefile is under 'powerpc'. You can temporarily create a 'symbolic link' from powerpc to ppc and resolve this.
Once the driver has compiled (assuming it now does) you can remove the link:
sudo ln -s /usr/src/linux-headers-3.0.0-15-powerpc/arch/powerpc /usr/src/linux-headers-3.0.0-15-powerpc/arch/ppc
Now, should there still be some problem please report the error exactly as its displayed. You might also attatch the 'install.sh' script so I can get some idea of what its doing. The more information you provide me the better I can see what the issue is ..
sudo rm -f /usr/src/linux-headers-3.0.0-15-powerpc/arch/ppc