Hi,

you need to copy the library to a "designer" subdirectory of your plugin directory. Especially during development, you can also use the QT_PLUGIN_PATH environment variable to point to the plugin directory (but you still need the designer sub directory). See also http://doc.qt.nokia.com/latest/desig...ing-the-plugin.
The "Help/About plugins" menu is quite helpful when plugins are not recognized by QtDesigner, since it also lists missing library dependencies. Also take care that your LD_LIBRARY_PATH contains all directories where additional dependend libraries might be located.

Hope this helps,

Andreas