Look on the edges where the sides connect to the main core. Mine was cracked along that and was difficult to find.
Also, the UView Airlift Cooling Leak Check and Flush system is the best 100 bones you will spend. This is what Pedro suggests in his DIY notes, so I went and ordered it. You can refill your whole car in less then 10 minutes without fear of having bubbles anywhere in the system. Here is a link to it my friend:
Amazon.com: UView 550000 Airlift Cooling System Leak Checker and Airlock Purge Tool Kit: Automotive