Open the NSTextField reference. (Developer Documentation in the Help menu). Enter NSTextField.
Look through the methods that are available. Anything like "title" or "value" or something like that that looks right?
If you can't find anything, look at the top if the NSTextField class reference where it says "Inherits from NSControl : NSView : NSResponder : NSObject". So try clicking on NSControl. Or look at "Companion Guide Text fields" and click on that.
You should find what you are looking for in the NSControl reference. Now that method is useful if you want to change or set the text when the program runs. If it is fixed, go into Interface Builder and look for a place to enter the property that you just found.