Can't find postgres.h or fmgr.h

I have installed the binary of version 8.0.2 using the installer, and I
requested all the developer options from the install but I can't find
postgres.h or fmgr.h I need to write custom functions.

 

I am new to postgres and to C so forgive my ingorance. And if I have posted
to the wrong list please let me know.

 

Thanks,

Mark