Hi there!
I'm not used to work with Interface Builder and it's elements (I mostly work with OpenGL and Cocos2D), but I've done some little things and I think I can help you a little.
You should have a method associated with every number. When calling this method you just simply need to append the associated number to the textfield content. It should be something like this:
Code:
-(void)touchNumber:(int)num
{
[self.textField setText:[self.textField.text stringByAppendingString:[NSString stringWithFormat:@"%d",num]]];
}
Note that you should have declared the textField as a property of your class so you can refer it from anywhere (inside the class, obviously).
With this you should append as many numbers as you want to the textField using your own buttons (when touching a button simply call the "touchNumber" method with the corresponding number).
Hope it helped, let me know if you have any other doubt and I will help you if I can!