xcode instance help!

No, the - represents an instance method and the + represents a static method.

I found a (working) copy of the tutorial that I did when I was learning Objective C, and it turns out that convertAmount:atRate: is a method of the Converter class, but the error you are getting suggests that the message is being sent to the ConverterController class. Check what objects you are sending the convertAmount:atRate: message to.

For reference, I've put a copy of the code from my copy below (hopefully it's the same version).

ConverterController.h:

Code:

/* ConverterController */

#import <Cocoa/Cocoa.h>

@interface ConverterController : NSObject
{
    IBOutlet id converter;
    IBOutlet NSTextField *dollarField;
    IBOutlet NSTextField *rateField;
    IBOutlet NSTextField *totalField;
}
- (IBAction)convert:(id)sender;
@end

ConverterController.m:

Code:

#import "ConverterController.h"
#import "Converter.h"

@implementation ConverterController

- (void)awakeFromNib {
	[[rateField window] makeKeyAndOrderFront:self];
	[rateField selectText:self];
}

- (IBAction)convert:(id)sender
{
	float rate, amt, total;
	amt = [dollarField floatValue];
	rate = [rateField floatValue];
		
	total = [converter convertAmount:amt atRate:rate];
	
	[totalField setFloatValue:total];
	[rateField selectText:self];
}

@end

Converter.h:

Code:

/* Converter */

#import <Cocoa/Cocoa.h>

@interface Converter : NSObject
{
}
- (float)convertAmount:(float)amt atRate:(float)rate;

@end

Converter.m:

Code:

#import "Converter.h"

@implementation Converter
- (float)convertAmount:(float)amt atRate:(float)rate {
	return (amt * rate);
} 
@end