I updated project to Xcode 5 and enabled modules in build settings.
However, I see compiler error Unexpected \'@\' in program when I use @import.
From the comment of @hw731 I think you use badly @import :
old syntax to import framework :
#import <UIKit/UIKit.h>
but now, you can use the new syntax :
@import UIKit;
you need to enable theses modules to use the keyword @import (it's enable by default when you create a new project with Xcode 5) :

Have a look here.
Got same error, checked the standard places:
The problem was that the header file was imported into file.mm file, which doesn't seem to support the new @import module syntax! Converted that header back to old style #import format and everything was fine again.
Same problem when i subclass UIActivity and write property in .m file as
@property(nonatomic, strong)NSArray *activityItems;
fixed by moving this property to .h file.
I also found that using following code in pch file:
#ifdef __cplusplus
#import <opencv2/opencv.hpp>
#endif
are not compatible with @import.