To connect to NUcat using Z39.50 you will have to configure your client with certain information. Northwestern's library system is from Endeavor Systems Inc., they also have additional information [1] on their site relating to Z39.50 access via Voyager. Each of the 4 databases in Northwestern's Voyager system has a slightly different configuration, making use of different port numbers and search fields attributes.
Some important notes about NU Voyager database acess via Z39.50
Links:
[1] http://www.endinfosys.com/new/z3950.htm