i do not know what is included in the openacs tarball, but if you want to compile the postgres driver for aolserver, you need some header files from the aolserver and postgres include directories. those are the files listed in your error message (ns.h, nsdb.h, nstcl.h, libpq-fe.h). this files should be available in "include" subdirectories of your aolserver and postgres installation dirs.
my way of installing is to compile everything from source (i would suggest using aolserver4 and nspostgres4.0) but maybe this is not the right way for you as a newbie. the fastest way to get things running for you is probably to have a look at the Makefile of your nspostgres source and enter the correct paths to aolserver and postgres include directories (this is where the *.h files are located). When those header files are found, compiling should be working alright.
hope that helps, peter
