No need to worry, Sophie. Everything is fine. Your travel itinerary fully qualifies for the 240-hour visa-free transit policy, so you can go ahead with your plans without any concerns. Since the places immediately before and after your mainland China visit are two different countries/eligible territories (Malaysia and Macau), and your total stay in mainland China will be less than 240 hours, you're eligible for this policy. When checking in at Kuala Lumpur, simply inform the staff that you'll be traveling to Macau via mainland China using the 240-hour visa-free transit and show them your flight ticket from Beijing to Macau. Upon arriving in Nanjing, the first step is to go to the 240-hour visa-free transit counter. There, you'll need to apply for a temporary entry permit by presenting your flight ticket to Macau and a brief itinerary of your stay. Once the permit is approved, go through immigration and then you're free to leave the airport and enjoy your time in China. Have a wonderful trip!