As far as I am aware the digital camera must have webcam functionality, I don't think that because it can stream video that it will be able to act as a webcam... DON'T QUOTE ME ON THAT!!!
Have a read of the manual...and see what it says.
My own digi cam just requires me to press the "mode" button and it automatical switches to webcam... see if your cam has the same sort of setup... another thing to consider is have you installed the drivers for the cam on your machine. If you're using Windows XP you don't need a driver for "Mass Storage" but you will need one for webcam finctionality.... at least that's how my camera works anyways.
Hope that helps.