setting target and action for a NSSlider programmatically in a class that is not "file's owner" - how?

FROM : Rua Haszard Morris
DATE : Fri May 16 04:12:43 2008

I'm trying to set the action for an NSSlider in a little helper class 
that is not the "file's owner" class associated with the window in the 
nib. At runtime, I get these console messages:

*** +[SliderHelper myAction:]: unrecognized selector sent to class 
0xeb74380
HIToolbox: ignoring exception '*** +[SliderHelper myAction:]: 
unrecognized selector sent to class 0xeb74380' that raised inside 
Carbon event dispatch

This is how I'm setting this up:
  - there's an NSObject subclass (call it MyDialog) containing 
IBOutlets for all the controls I care about.
  - there's another NSObject subclass (call it SliderHelper) which is 
intended to automate some behaviour common to a few controls in the 
dialog (and other dialogs in future)
  - MyDialog instantiates a SliderHelper and passes it an NSSlider* 
outlet referring to a slider
  - SliderHelper sets up the target and action for the slider 
programmatically in an +initWithSlider: method..

+(id) initTestSlider:(NSSlider*)aSlider {
   SliderHelper* newOne = [[SliderHelper alloc] init];
   [newOne setSlider:aSlider];
   [[newOne slider] setTarget:self];
   [[newOne slider] setAction:@selector(myAction:)];
   return newOne;
}

  - myAction: is a void returning method with no params, and it is 
implemented!

How can I make this work?

Note that if I set the target and action from MyDialog (with a 
MyDialog -myAction of course) it works, what am I missing here?

thanks
Rua HM.