get mouse event in cocoa window controller

I would recommend subclassing NSTextField and handling the events there. As trojanfoe said, it has mouse-handling built in. Plus the feature you describe sounds like something that you might use again, in the same app or another. Just setting the class to your custom NSTextField will save time.

It could look something like this:

DCOHoverTextField.h

#import <Cocoa/Cocoa.h>

/** An `NSTextField` subclass that supports mouse entered/exited events.
 */
@interface DCOHoverTextField : NSTextField

@end

DCOHoverTextField.m

#import "DCOHoverTextField.h"

@interface DCOHoverTextField()

/* Holds the tracking area for the `NSTextField`. */
@property (strong) NSTrackingArea *trackingArea;

@end

@implementation DCOHoverTextField

- (void)updateTrackingAreas {
    // Remove tracking area if we have one
    if(self.trackingArea) {
        [self removeTrackingArea:self.trackingArea];
    }

    // Call super
    [super updateTrackingAreas];

    // Create a new tracking area
    self.trackingArea = [[NSTrackingArea alloc] initWithRect:self.bounds
                                                     options: NSTrackingMouseEnteredAndExited | NSTrackingActiveAlways
                                                       owner:self
                                                    userInfo:nil];

    // Add it
    [self addTrackingArea:self.trackingArea];
}

- (void)mouseEntered:(NSEvent *)theEvent {
    // TODO: Change text field into a plus sign.
}

- (void)mouseExited:(NSEvent *)theEvent {
    // TODO: Change text field back into a regular text field.
}

@end

After creating your subclass, go into Interface Builder, select your NSTextField and change the class to the subclass you created.