Wrong syntax for the connect() statement. Should be:

Qt Code: 
  1. connect(d_knob, SIGNAL(sliderMoved(double)), this, SLOT(testQwtDialSliderMoved(double)));
To copy to clipboard, switch view to plain text mode 

and be sure to define "testQwtDialSliderMoved()" as a slot in your MainWindow class.

Please use CODE tags when posting code. Click "Go advanced" and then the "#" icon to insert the open/close tag pair. Paste your code between them.