That could be why you're having problems
The code that you've posted should go into the .m file. Headers contain interface code while .m files contain the implementation details.
Looking at your code, I'm guessing that your header file should contain something like the following
Code:
@interface AppController : NSObject {
IBOutlet id textBox1;
}
- (IBAction)add3:(id)sender;
- (IBAction)subtract3:(id)sender;
@end