You can customize the quoting character, by following the following steps.
- On the PgAdmin-III main window, click on File and then Options
- Click on the Query Tool Tab
- Change the field called "Result copy quote character" to apostrophe (')
It should work, you may need to close and open PgAdmin-III
EDIT: For Mac OS X the option could be found in pgAdmin3 > Preferences > Query tool > Results grid