-(void) updateApplicationIcon:(int) theNumber:(NSRect) theRect{
	// get the current app icon
	NSImage *appImage = [NSImage imageNamed:@"NSApplicationIcon"];
	
	// create a new image and draw the app icon onto it
	NSImage *image = [[NSImage alloc] initWithSize:[appImage size]];
	
	// lock focus on the new image and draw the app icon onto it.
	// you'll want the image flipped so text shows up correctly
	[image setFlipped:TRUE];
	[image lockFocus];
	NSSize appSize = [appImage size];	
	[appImage compositeToPoint:NSMakePoint(0, appSize.height) 
					 operation:NSCompositeSourceOver];
	
	// draw red circle
	NSBezierPath *redCircle = [NSBezierPath bezierPathWithOvalInRect:NSMakeRect(8,43,45,45)];
	[[NSColor redColor] setFill];
	[redCircle fill];
	
	// draw whatever else you want onto the image
	NSArray *values = [NSArray arrayWithObjects:[NSColor whiteColor], [NSFont fontWithName:@"Tahoma" size:32], nil];
	NSArray *keys = [NSArray arrayWithObjects:NSForegroundColorAttributeName, NSFontAttributeName, nil];
	
	NSDictionary *attrs = [NSDictionary dictionaryWithObjects:values forKeys:keys];
	NSString *caption= [NSString stringWithFormat:@"%d", theNumber];
	NSAttributedString *text= [[NSAttributedString alloc] initWithString:caption attributes:attrs];
	[text drawInRect:theRect];
	
	// unlock focus and set the new image as the dock icon
	[image unlockFocus];
	[NSApp setApplicationIconImage:image];
}

-(void) updateApplicationIcon{
	// get the current app icon
	NSImage *appImage = [NSImage imageNamed:@"NSApplicationIcon"];
	
	// create a new image and draw the app icon onto it
	NSImage *image = [[NSImage alloc] initWithSize:[appImage size]];
	
	// lock focus on the new image and draw the app icon onto it.
	// you'll want the image flipped so text shows up correctly
	[image setFlipped:TRUE];
	[image lockFocus];
	NSSize appSize = [appImage size];	
	[appImage compositeToPoint:NSMakePoint(0, appSize.height) 
					 operation:NSCompositeSourceOver];
	
	// draw red circle
	NSBezierPath *redCircle = [NSBezierPath bezierPathWithOvalInRect:NSMakeRect(8,43,45,45)];
	[[NSColor redColor] setFill];
	[redCircle fill];
	
	// draw whatever else you want onto the image
	NSArray *values = [NSArray arrayWithObjects:[NSColor whiteColor], [NSFont fontWithName:@"Tahoma" size:32], nil];
	NSArray *keys = [NSArray arrayWithObjects:NSForegroundColorAttributeName, NSFontAttributeName, nil];
	
	NSDictionary *attrs = [NSDictionary dictionaryWithObjects:values forKeys:keys];
	NSString *caption= [NSString stringWithFormat:@"%d", newMessages];
	NSAttributedString *text= [[NSAttributedString alloc] initWithString:caption attributes:attrs];
	NSRect theRect = NSMakeRect(22,36,50,50);
	if (newMessages > 9){
		theRect = NSMakeRect(13,36,50,50);
	}
	[text drawInRect:theRect];
	
	// unlock focus and set the new image as the dock icon
	[image unlockFocus];
	[NSApp setApplicationIconImage:image];
}

El texto en blanco viene definido por el parámetro theNumber.
La posición del texto viene definido por el parámetro theRect

Añade un círculo rojo con un texto en blanco al icono de la aplicación en el dock

Objective C