To begin - navigate to Catalog -> Improved Navigation then click on Filters tab. Here you can see all the filters that are available. If you recently added a new attribute click the "Load" button to load it in (Upper right of that screen). Note: More general configuration settings can be found under the Settings Tab.
To modify a Filter, click it on the grid.
Show in the Block setting will determine where the filter is shown. We have this set to "Top".
Display list will determine how the attribute will be displayed
Hide Quantities will hide/show the number of products matching certain filters that are selectable.
Sort Options By will determine how the sorting is done within the navigation.
Include only in categories determines if the attribute filtering is only going to be shown in certain categories.
Exclude From Categories does the opposite.
Single Choice Only forces the user to only filter one attribute at a time if you are using multiple attributes for filtering.