You should import the resources-iphone, and resources-iphonehd directories in your XCode project as folder references instead of groups. You can do that by drag and dropping the folders onto your XCode project. When you do that you will see the directories appearing in your applications bundle. You can check that by selecting your application's target and going to Build Phases->Copy Bundle Resources.
After that, on you ApplicationDelegate.cpp file you should add those folders in the search paths list of CCFileUtils.
std::vector<std::string> resDirOrders;
if (resolution is iphone)
{
resDirOrders.push_back("resources-iphone");
}
else if (resolution is iphone retina)
{
resDirOrders.push_back("resources-iphonehd");
resDirOrders.push_back("resources-iphone");
}
CCFileUtils::sharedFileUtils()->setSearchResolutionsOrder(resDirOrders);
This is the recommended way of handling multiple resolutions according to the cocos2d-x devs (check out this article). You can use the window size from the CCDirector to determine the current resolution. In the iphonehd case I'm including the non-retina folders as well as a fallback.