Click on the "Search" button (see screenshot below), and you'll be presented with the Search Dialog box. Here, you can put in criteria to narrow the list of displayed output styles.

For instance, if you know the in-text citation style, the bibliographic sort order, and the first three or four fields that are displayed in the bibliography for a Journal, you can fill this information in and click on the "Search" button to get results that only match those criteria.

Additionally, if you click on one of the listed output styles, you will see a preview of what a reference would look like formatted with that style. This is very useful in trying to find a close match for an output style that isn't currently provided by Reference Manager.