Once you are in the frame-grabbing window of the video toolbox, you will first need to choose whether you would like to grab a frame from the captured video or from the camcorder. Once you have made that selection, and the frame you want is being shown in the preview window to the right you can press the "Grab" button. The frame will move to the smaller preview window to the left. Below the smaller preview window you can then choose to "Add to movie" or"Save to disk". Selecting Save to disk will bring up a standard Windows file save box where you can choose the size, type, and location of the image you have captured.
More detailed directions for the frame grabbing tool can be found in your Studio manual.