How and were can I add a curled drop shadow to an web element

How and were can I add the following curled drop shadow to an web element such as the background of a sitebar. http://gmcpro.com/presentation/images/video-shadow-box.png? I’m working in wordpress.

I see a lot of generators online that give me the code but were and how do I place it in what css of php?

I hope someone will help me