Some Xcode help needed.

It's probably something like this

Code:

 NSSTring * str = [input getValue]; //or getString, or ... (see docs)
 [label setStringValue:str];

However, this is much nicer, cooler way to do this.
Add a NSString *str instance in the header of your class. Bind the value of both the input text field and the label to str. Your label will change automatically without the need for a button. It's called KVC (key value coding). Very useful to learn.
If you want to change the input before displaying it, you overwrite the set and get functions of KVC. Read up about KVC for more information

Code:

-(NSString *) getStr;
-(void) setStr:(NSString *) value;

Unless of course, the user needs to explicit take action. Then you still need the button.