I would upgrade to pg 7.0, but that's probably not your problem here. However I think you do need the new as-pg driver. I think Roberto said something about aolserver using our driver for postgres, but I'm not sure about that. Anyway when you compile your driver, your should see something like the following:
gcc -DFOR_ACS_USE -I/usr/local/aolserver/aolserver3_0/include -Wall
-fpic -shared -I/usr/local/pgsql/include -I/home/nsadmin/include -I-/usr/include -c postgres.c -o postgres.o
gcc -shared -I/usr/local/include -I/usr/local/aolserver/aolserver3_0/include -I-/usr/include -o postgres.so postgres.o /usr/local/lib/libpq.so
The key is the -DFOR_ACS_USE symbol definition. Ns_DbAddColumnInfo is conditionally defined using FOR_ACS_USE. In our driver make file FOR_ACS_USE is defined as a CFLAG option by default. Check your driver makefile.