that is because you need to install the web plugin. usually when you use a webcam with a website based video chat system, you need to install an internet plugin that lets the browser connect with your webcam. you can usually get this when you go to the website and it says that you are missing a plugin, you would then click on anything that says "get this plugin"" or something of the sort and then it takes you to a separate site to download it. after that it should work when you go back to the website.
P.S. if you are on a pc, then you would see a puzzle piece in the space where the video chat would be. i am not sure what a mac would have. most likely something similar. But macs have webcams already setup for those kinds of things