How to add a lib to a qt project

[wysota]

I want my qt App communicate with a postgresql database.
Chris and stampede told me I have to build a psql plugin for qt.

And you intend to do it by compiling a project from a directory that doesn't contain any source (nor other project) files? You expect some magic to happen that your compiler knows how to build that plugin from an empty directory?

The psql-folder was just created by me.

You shouldn't be creating any directories. Qt sources should contain a psql directory inside src/plugins/imageformats and there should be a bunch of files there including a README file containing requirements and instructions for building the plugin.