The URL where your catalog is located. Do not include anything after the hostname.

Correct: http://addison.vt.edu

Incorrect:
http://addison.vt.edu/
http://addison.vt.edu/search
http://addison.vt.edu/search?
http://addison.vt.edu/search~S5 (set 'Catalog Search Scope' to 5 instead)