How to create an NSEvent of type NSScrollWheel?

You can create a CGEventRef using CGEventCreateScrollWheelEvent and convert it to an NSEvent with eventWithCGEvent:. To add information to the CGEvent, use CGEventSetIntegerValueField -- among the possible fields are kCGScrollWheelEventDeltaAxis1, ...2, and ...3.

Try following code:

  int scrollingDeltaX =... //custom value

//if you have some NSEvent *theEvent 
  CGEventRef cgEvent = CGEventCreateCopy(theEvent.CGEvent);

  CGEventSetIntegerValueField(cgEvent, kCGScrollWheelEventDeltaAxis2, scrollingDeltaX);

  NSEvent *newEvent = [NSEvent eventWithCGEvent:cgEvent];
  CFRelease(cgEvent);

NSEvent doesn't provide a way to do this, but you can create a CGEvent and create an NSEvent wrapper around it. The event will automatically use the current location of the cursor. See CGEventCreateScrollWheelEvent. However, posting this event using NSApplication's postEvent:atStart: doesn't work (probably because it doesn't have a window defined). You can either post the CGEvent directly to the event stream, or send the NSEvent directly to a view's scrollWheel: method.

CGWheelCount wheelCount = 2; // 1 for Y-only, 2 for Y-X, 3 for Y-X-Z
int32_t xScroll = −1; // Negative for right
int32_t yScroll = −2; // Negative for down
CGEventRef cgEvent = CGEventCreateScrollWheelEvent(NULL, kCGScrollEventUnitLine, wheelCount, yScroll, xScroll);

// You can post the CGEvent to the event stream to have it automatically sent to the window under the cursor
CGEventPost(kCGHIDEventTap, cgEvent);

NSEvent *theEvent = [NSEvent eventWithCGEvent:cgEvent];
CFRelease(cgEvent);

// Or you can send the NSEvent directly to a view
[theView scrollWheel:theEvent];