The Log window only displays the "currently selected" entries. This current selection may be a single QSO or your entire log. When you open a log no entries are shown until you make a select entries using the Select and Search buttons as described below. After a search the first entry that satisfied your search will be displayed and the number of entries selected will be shown in the status bar. If the selection contains more than one entry you can use the scroll bar or the mouse wheel to move through them.
To select all log entries choose Select and then Search without entering any search keys.
To select a subset of your log entries choose Select, enter search keys that describe the entries you want, and then choose Search. For example, to select all CW QSOs with Brazilian stations choose Select, set the Mode field to CW, set the Prefix field to PY, and then choose Search.
After a search you can choose one of the Display menu items if you want to print or view the currently selected entries as a list.
Menu items like Display/Short give you the choice of only including the currently selected log entries or all entries in your log.
For more information on searching techniques see Search Keys and Wildcards in Search Keys.
Normally entries selected by Search are sorted by date and time. To sort on other fields (like prefix or call) choose Display/Sort By. Note that changing the sort order only applies to subsequent searches and does not affect the currently selected entries.