To fill in any red, unfilled positions, first select that position from the schedule.  Make sure that the right side of the screen is set in the "Fill Positions" mode.

The list will filter out who is available to be scheduled at that shift.  When you click on a volunteer's name, if there is a conflict, or a reason VSP could not place them in that position, it will be displayed at the bottom of the list in red.  This is a good way to answer the question "Why is this position left unfilled?"

You can double-click on a volunteer's name, or drag and drop them from the list, to place them in the selected position.  

To expand the filter and see a broader list of volunteers to choose from, click on the text at the top of the "Fill positions" menu that says "Show only volunteers who..." and this will open a drop-down menu with the different filters you can turn on or off by selecting them.