The steps above can be followed to add any database in MarcEdit. For a list of supported databases, follow the Library of Congress' link below:
List of Z39.50 Servers
NOTE: Support for searching with Z39.50 varies by institution, so you may be able to configure MarcEdit to search an institutions database, but if they do not support Z39.50 any longer or require extra permission to search their collection, then you will not know this until you attempt a search. A notable example is OCLC, which requires certain permissions to use Z39.50.