How to view data in pgAdmin III

Or, to retrieve all rows, just press the blue data grid.

The tooltip "View data in the selected object" is almost making it too easy.
Try the button with the funnel next to it to retrieve only a selection (for big tables).

Right click on the table and choose "View data"

For a list of alternatives, check out the Postgres wiki:
http://wiki.postgresql.org/wiki/Community_Guide_to_PostgreSQL_GUI_Tools