Hi there !
I've encountered the same problem You did and I've solved it in the next way: I've downloaded ZeosLib database access library from sourceforge.net/projects/zeoslib/, it has a package for Lazarus and
downloaded PostgreSQL database server from
www.postgresql.org/. PostgreSQL is a free database
server and it has precompiled binaries for Windows and Linux and also You can download the source too.
Under PostgreSQL if You want to execute a stored procedure just simply open a ZQuery object with
following statement: "select StoredPorcedureName(Input1, Input2,...)". After opening You will get a
table having in the fields the output parameters of your stored procedure

Simple

Have fun !
Zoltán