you have to insert the jar of the driver JDBC in the classpath.
It would be something like that:
java -cp pgsqlxx.jar;miapp.jar com.project.Main
or maybe:
java -cp pgsqlxx.jar -jar miapp.jar (if the manifest og myapp.jar is correct and include the Main class). (
http://java.sun.com/j2se/1.4.2/docs/...#37;20Manifest )
good luck