IOS Hookクラスメンバー変数

It seems that in your case you are trying to use an instance variable of the class you are hooking. Modifying the instance variable does not work that way in tweaks. You have to use MSHookIvar to 'hook' an instance variable (aka ivar). Example:
[Tweak.xm/mm]

#import <substrate.h> // necessary

#import <Foundation/Foundation.h>



@interface TheClassYouAreHooking : NSObject {

    NSString *_exampleVariable;

}

- (void)doSomething;

@end



NSString *_exampleVariableHooked;



%hook TheClassYouAreHooking

- (void)doSomething 

{

    // 'Hook' the variable



    exampleVariableHooked = MSHookIvar<NSString *>(self, "_exampleVariable");



    // The name of the hooked variable does not need to be the same



    exampleVariableHooked = @"Hello World";



    // You can do ANYTHING with the object Eg. [exampleVariableHooked release];



}

%end

　　
MSHookIvar can also hook stuff like BOOLs and floats etc.

exampleVariableHooked = MSHookIvar<BOOL>(self, "_someBOOL");

　　
Its declared in substrate.h so you need to import that otherwise you will not be able to compile your tweak. Also as a bonus tip, I'm just reminding you that you have to put the identifier of the app/framework you're hooking in your tweakname.plist.
So after you 'hook' the variable you can change it to suit your needs. Happy coding!