Assuming its something like a Logitech cam, see the attached images in the following order.
1) start.png (On the left side, pull the vertical bar out so you can see the sources list. Click on devices to expand, select webcam...assuming its compatible)
2) double-click.png (Once your cam is selected, double click it to make the screen appear like this. Then click the universal symbol for play. |> )
3) viewer.png (Assuming your going to need to see if the camera is working, Output>Show Preview will be the next thing to do.
For help with Ustream (when it's not down like today), go here.
Remember that your Flash camera source is vloopback, NOT the webcam when at the ustream streamer. Browser must be closed when starting and setting up WebcamStudio. Start browser only after step three above.