To get your app to see the PG driver, you can use the Add Library, but to
get the *IDE* to see it for design, make a file called Postgres.config in
/.../JBuilderX/lib/ext with the line
addpath C:\Documents and Settings\Andrew
Lazarus\jbproject\classes\pg74.1jdbc3.jar
(obviously modified for your own paths; works on Unix with forward slashes).
The pay versions of JBuilder are supposed to generate the config file when
you add the library, but the free one won't.