Open one Form (widget)

Hello for all o/

err... I have one class called:
class Ui_Form_user{}; This class

And I´ve one mainwindows and one action called: action_cad

I want help for make one fuction for open (press one option for menu) widget when action_cad actived.

Help me plx

I do not understand the question. Look at these tutorials and the various examples that ship with Qt.

http://qt.nokia.com/doc/4.5/tutorials.html

There is for sure examples that use more than one window and show how to proper connect them together.