The userData property is initially nil. You have to create a dictionary and assign it first:
self.testSprite.userData = [NSMutableDictionary dictionary];
[self.testSprite.userData setValue:@"CAT" forKey:@"key"];
NSLog(@"%@", [self.testSprite.userData objectForKey:@"key"]);
NSLog(@"%lu", [self.testSprite.userData count]);