In PL/SQL Developer 6.0, you can go to Tools > Preferences > Editor and enable the "Highlight parentheses" option.
For the Program Window there is additionally a preference to highlight:
- if / then / else / elsif / end if
- loop / end loop
- for / loop / end loop
- while / loop / end loop
- case / when / then / else / end case
- declare / begin / end