if you don't have the nspostgres.so driver, download nspostgres4.0beta1 from aolserver.com's download section and compile. there are instructions included in the tarball, but basically use the command:
% make ACS=1 POSTGRES=/usr/local/pgsql NSHOME=/usr/local/aolserver
% sudo make install
if you are using aolserver 4...
aolserver 4 has broken the database stuff into an external module.
add to your nsd.tcl file:
ns_param nsdb ${bindir}/nsdb.so
for reference:
https://openacs.org/forums/message-view?message_id=137219